Introduction To North American Railway Signaling 2008928725, 9780911382570

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Introduction To North American Railway Signaling Compiled by: Kendrick Bisset, Tony Rowbotham, Dave Thurston, Rob Burkhardt, Jeff Power, and Jim Hoelscher for the North American Section of the Institution of Railway Signal Engineers

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North American Section Simmons-Boardman Books, Inc. 1809 Capitol Avenue Omaha, NE 68102

800-228-9670

www.transalert.com

First published 2008

ISBN 978-0-9 11 382-57-0 Library of Congress Control Number: 2008928725 Copyright 2008 Institution of Railway Signal Engineers All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, photocopying, recording or otherwise, without the prior permission in writing of the Institution of Railway Signal Engineers. The information presented in this book is in no way intended to supersede or negate any rules or regulations of government bodies, the AAR, or individual carriers. Further, it is not intended to conflict with any currently effective manufacturers operating, app lication, or maintenance instructions and/or specifications. The publisher is not responsible for any technical errors that might appear. Printed in the United States of America by Simmons Boardmam Books, Inc.

Front cover picture : End of siding interlocking in the Rocky Mountains. The signal is displaying red over green for a move into the siding with the switch lined for the diverging route. A slide fence is on the left of the track.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v 1.

Railway Signaling . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. 3.

The Elements of a Signal System . . . . . . . . . . . . . 9 Track Work ........ .. ... . . . .... .. .... . ... 23

4.

Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5. 6. 7.

Wayside Signals ........ .. . .......... . .... 51 Relays and Relay Logic . . ..... . ..... . .... . . 67 Train Detection ... .. ........... ........... 85

8.

Block Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

9. 10.

Interlocking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7 Remote Control and Control Centers . . . . . . . . . . 133

11.

Cab Signals and Automatic Train Control . . . . . . 141

12.

Grade Crossings ......... . . . ... . . . . . . . . . .. 155

13.

Defect Detectors . . . .. . . .. .. ............... 167

14.

Lightning and Surge Protection . . . . . . . . . . . . . . 191 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 199 Glossary .. ... .... ......... . .. .. ... ..... . 203

Foreword

I am pleased to be able to provide a brief foreword for this, the first, publication from the North American Section. This book is intended to provide an introduction to and the background of practices appli ed in North America.

I am sure it will provide a usefl.il reference for new beginners to the signaling industry in many disciplines and provide an insight to signaling engineers globally of such practices.

I hope this will be the first of many publications to support and develop the industry in North America. May I record the Institution's sincere thanks to all who have contributed in any way to this reference book.

A.J. Fisher P resident Institution of Railway Signa l Eng.ineers May 2008

111

Introduction

K. D. G. Bisset

v

Introduction

Introduction The rai lroad is a very efficient transportation system. The use of steel wheels on steel rai ls yields a very low rolling resistance, and the capability of carrying very heavy loads. Today, one fre ight car can carry over I 00 tons of cargo, yet even loaded, one or two people can move the car without power assistance. (True, it is not easy, it is very slow, and a lever device is necessary, but it is poss ible.) A fre ight train can have more than I 00 cars and be over a mile long; a typical onboard crew is two or three people. To carry the heavy loads, the equipment is very heavy. An empty freight car might we igh 20 tons. A rail wi ll weigh about 130 pounds per yard. This means the equipment is also expensive, not only in first cost but also in continuing maintenance. The land on which the track is built is owned by the railroad, which means initial purchase expense and continuing taxes. These factors mean that everything must be used efficiently or the railroad w ill lose money and go out of business. The steel wheel on stee l rail cannot sustain large accelerations or decelerations. Braking distances (the distance required to stop) may well be over a mile at speeds as low as 50 mph (miles per hour). The braking di stance increases dramatically with increasing speed. Thus, a railroad has the following features: • Trains cannot steer around obstacles. • Multiple trains must share the track. • Braking distances are generally much longer than the distance that the train operator can see.

If the railroad operates perhaps only two or three trains a day, it is not difficult to prevent collisions between trains si mply w ith operating procedures. Even more frequent traffic can be controlled in this manner if the railroad (or a section of railroad being operated essentially separately) is short. One example of this type of operation is called in England " one engine in steam" (i.e., only one locomotive is in operation, so only one train can be on the railroad at a time). Likewise, if operation is very slow, operating procedures may be sufficient to avoid collisions. Lightly used branch lines and many short line ra ilroads do operate with these methods. This is called dark territo1J1, where there are no signals to "light the way." About 40 percent of North American railroad trackage has no signaling. These types of operation, however, do not make full use of the track and other fac ili ties . A signal system will allow more trains to operate at higher speeds over the same trackage. In February of 2001, the Institution of Railway Signal Engineers (IRSE) issued a report titled "IRSE Signalling Philosophy Review," reviewing the principles of signal design in the United Kingdom (UK). In many respects, North American signaling follows the UK approach. The IRSE Review contains a section titled "Why do we need signalling systems" (using the British spelling of signalling), which appl ies very well to North America. The follow ing are excerpts from that Review, with slight modifi cations in spelling and words to better match North American usage.

V il

Introduction

The overall twin purposes of s ignaling systems ... can be summarized as follows: The purpose of a signaling system is to facilitate the safe and efficient movement of trains on the railroad. Safety and efficiency, both of which are mentioned in the statement of purpose, do not always sit easily with each other. The fundamental safety requirements of a signaling system include keeping trains adequately separated from each other, and stopping (or slowing) trains where necessary to avoid potentially unsafe situations. Efficient operation of the railroad, on the other hand, is mainly about sending as many trains as possible along a given portion of track, as quickly as possible, using the minimum of infrastructure. The main function of a signal system is therefore to set up a route for the passage of each train over the track that it is to traverse, authorize the engineer (or train operator) to make the movement, maintain the route while the train is making its movement, and finally release the route (for use by other trains) after the passage of the train. The basics of signaling philosophy and signa ling techniques have their roots in main-line railroad. However, the signaling techniques have been developed and modified to meet the needs of various steel wheels on steel rail modes of transportation. In addition to freight service, these include main-line passenger service, commuter rai l passenger service, light rail transit, and heavy rail transit. Each of these modes has different speed, train density, train frequency, and train-size requirements that have resulted in the development or modification of specific signal ing techniques. The signal ing techniques, in general, can all trace their origin to the basic signaling principles discussed in the foll owing chapters. Where it is pertinent, mode-specific effects on basic signa ling principles and techniques are described .

About This Book T here are precious few books on railroad signa ling. The Institution of Railway Signal Engineers (IRSE) has produced two books on British practice, and a book summarizing European practice. In 1999, the IRSE released Introduction to Signalling, again on British practice (as implied by the spelling). Several chapters in this book were written by Tony Rowbotham, an employee of ALSTOM Signalling Limited in Borehamwood (near London), England. Books on American signal practice are even more scarce than those on British practice. Three of relatively recent vintage come to mind: 1. Elements of Railway Signaling. General Railway Signaling Co., 1979; a reprint of a work first published in 1954. 2. The Search for Safety. Union Switch and Signal Co., 198 1. 3. Automatic Train Control in Rail Rapid Transit. Un ited States Congress Office of Technology Assessment, 1976. The arrangement and topics covered in Introduction to Signalling suggested that it might be adapted, in part, to American practice. This present work is in part a translation from British practice and terminology to American practice and terminology. It is a fa irly basic coverage of the topic; more details may be obtained from manufacturers' literature and the books mentioned above.

Vlll

Chapter 1

RailY1ay Signaling

A. J. R. Rowbotham K. D. G. Bisset

Introduction

Introduction The signaling system is an essential part of the railway. Its principal task is to ensure that trains run safely, (i .e., one train must not run into the back of another, or two trains must not collide when both are approaching a junction). The signaling system, however, also affects the type of service that can be run on a section of railway. If there is a need only to run one train an hour, the signaling system wou ld be quite different from a system where there is a need to run trains every 2 minutes, as on a city rapid transit system. The place of signal ing and its development should be considered in the overall context of railroading, and the developments that affected the industry as a whole. One of the more important developments in railroading with respect to signaling in North America was the automatic air brake, invented by George Westinghouse in 1869. With improvements, the air brake was in nearly universal use by 1900, an extremely important advance in the control of trains. This ability to control the train had a major impact on the way signal systems were designed in North America, and may well have led to the assumption that the train would stop at a red signal. T he February 2001 report issued from the Institution of Railway Signal Engineers (IRSE), titled "IRSE S ignalling Philosophy Review," included a section titled "The Role of a Signall ing System Within a Railway" (using the British spelling), which is quoted here with slight modifications in spelling and terminology: Across the world there is a wide range of signaling systems in use, and the signaling principles underpinning them are not all the sa me. Even in the UK, there are differences of princ ip le between, for example, semaphore and color light signaling systems. [This applies in North America: there are differences in principle between different railroads. In most cases these differences are subtle, but they are present.] More fundamental ly, there is no absolute definition of the scope of a signaling system, in terms of what function s it has to perform. The widest definition of a signaling 'system' inc ludes not only the signaling equipment, but also people such as tower operators and train operators who interact with the equipment, and the procedures and rul es they have to apply. The allocat ion of signaling functions into two groups, namely those that are to be performed by the equipment and those that are to be performed by people, is not the same on every railway. The role of a signaling system may well involve more than just controlling the movement of a train from one location to another. A real part of the fac ilitation of train movements involves, for instance, the provision of management information relating to train movements, and the provision of track access and trackworker protection for maintenance purposes. Modern signaling systems can fulfil these roles. Even more broadly, the signaling system is but one part of the total railway, and in defining a set of requirements for a system, it is important that there is c larity regarding its role within the overa ll operation o f the railway. lt is becoming ever more apparent that an integrated systems approach is needed to the design and operation of the railway, in order that there is clarity about how responsibility is apportioned between the various clements. This is particularly true in the field of safety and signaling systems ....

3

1 - Railway Signaling

Background When railways first started in England (before 1840), there were no fixed signals and Policemen wandered around the station, changing the switches at junctions and giving instructions to the drivers (train operators) by colored flags by day and oil lamps by night. There was no means of conununication with the adjacent stations so there was no way of knowing whether it was safe to let a train leave for the next station. All that could be done was to give the Policeman an egg timer and a train was only all owed to leave a station a fixed time interva l after the prev ious one (notwithstanding the fact that the previous train might have stopped around a curve just out of sight). There were amazingly few accidents, probably because train speeds were not very high at that time. Very soon fixed signals on posts began to appear, which consisted of a board, with different railway companies using different shapes. These boards were pivoted so they could be turned end-on to the driver. If drivers could see the board, they had to stop at it. If it was turned end-on so that it was nearly invisible, then they could proceed. Signals of this principle can still be seen in France today on minor lines.

The lack of a positive PROC EED display was not very satisfactory so the next development was to have a positive indication for both the STOP and PROCEED indications, or aspects as they became known. The most famous version of this improved signal was the Disc and Crossbar where a disc being visible gave the PROC EED aspect, or a rectangular crossbar being visible, the STOP aspect.

4

Backgound In the United States, the ball signal was developed. A lightweight ball was hung from a pole on a rope. The ball hoisted to the top of the pole gave the PROCEED aspect, and a lowered ball was the STOP aspect. This arrangement (purportedly) yielded a term sti ll in use today: highball for a PROCEED signal.

In 1841 , however, the first of a new design of signal, the semaphore was introduced and this soon became the standard mechanical signal and is still used today. Based on a French Navy communications system, this early signal had an arm that was horizontal for the STO P aspect, inclined downward at 45 degrees for the CAUTrON aspect and downward at 90 degrees for the PROCEED aspect.

The next change led eventually to a most important deve lopment. Someone had the idea of controll ing the switches and signals from a central point at the station. Levers were connected to the signals by wires or pipel ines and to the switches by pipelines. This meant that the Policeman no longer had to wander about, as all the controls were concentrated in one place, thus inventing both the tower and the towerman. Quite often in the early days, the signal s sprouted from the roof of the tower relying on the driver's knowledge of exactly where to stop for each signal. Later the signals were placed at the point where the train should stop if required. This grouping, however, led to an even more essential development. Someone had the idea of connecting the levers together in such a way that they were physica lly locked unless it was safe to pull them. For example, signal levers could not be pulled unless the switch levers were set the correct way, or levers for two conflicting routes could not be pu lled at the same time. This feature was known as interlocking and is still one of the foundations of signaling today. The first rather crude trial was at Kentish Town in

5

1 - Railway Signaling

England in 1860. Initially, the interlocking features were somewhat rudimentary, but by the 1880s, fu ll comprehensive interlocking was a legal requirement for all stations in the United Kingdom. During this same period, simp le electrical communication between stations became possible, not yet telephonic, but initially by bells using a Morse-code-li ke system. This required an electrical connection along the trackside: the telegraph wires and poles. This, in turn, led to another important development- the block system . Th is was a system of communication between stations, which, when used under a very strict procedure, ensured that only one train can be on the track between one signal box and the next at any one time. This section of track was known as the block and the system known as the absolute block or manual block system. Again by the 1880s, the abso lute block system was a compul sory legal requirement for all passenger lines in Britain. The next important development, in 1872, was the track circuit-electrical circuits actually in the rails that could detect, in a safe manner, that a section of track was not occupied by any rail vehicles. In the United States, the labor cost of manua l block operation was quite high, and so its use was limited. If track c ircuits were installed completely covering a section of plain track, automatic signals could be introduced, worked solely by the operation of the track circuits, no action by the towermen being necessary. One of the first of such install ations used compressed air to work the semaphore signals, the ai r being controlled by electrical valves worked from the track ci rcuits. In time, this type of signal ing became known as automatic block signaling, the "block" now being the space between signals rather than the space between towers. The track circuit had a wide-ranging impact in many areas of rai lroad signaling. The track circuit was the first embodiment of the closed circuit principle, and led to the fail safe design in other circuits. T he relay used for the track circuit had to be deve loped to match the principles of the track circuit design, which led to the vital relay designs in use today. The application of the track circuit allowed electric locking at interl ockings, and eventual ly the development of all-relay (and subsequently microprocessorbased) interlockings. It can be said that the track circuit is the heart of present-day signaling. During this period, more and more use was being made of electricity. Electric switch machines were introduced to operate the track switches. This re lieved the towermen of a lot of backbreaking work. The amount of physical effort needed was quite large, and to protect towermen, a practical limit was set on the distance that switches could be from a tower. With the introduction of electric switch machines, the limit was irrelevant and the area controlled by one tower could be increased. Electric machines were also introduced to work semaphore signals, although the physical effort needed to work signals was far less than that needed for switches. Semaphore signals were introduced that displayed aspects in the right-hand upper quadrant; that is, the arm projected to the right of, and perpendicular to, the mast for the STOP indication, and inclined upward at 45 degrees or 90 degrees for the CAUTION or CLEAR indication, respectively. This configuration is memorialized today in the symbols used for signals. Also during this period, more and more of the interlocking was carried out using circuits involving electromechan ical relays. By now the mechani cal locking between levers was very soph isticated and in a big interlocking, with, say, 200 levers, the apparatus would take up two stories of the building. The introduction of relays meant this space requirement could be reduced. In 1914, the first color light signals using colored lenses and electric lamps rather than mechanical semaphore signals were introduced on a main line. Different colors gave different instructions to the train

6

Background

operators. Semaphore signals, of course, could not be seen at night and were fitted with oil lamps shining through co lored glass for nighttime use. Electric lamps were much brighter and gave the same indications by day and by night. In 1912, standards were laid down for the meanings of the co lors that have remained in use today. With the introduction of color light signals and electric switch machines, the long levers with their big mechanical advantage were no longer needed when all they were doing was to work electric switches. Interlocking machines with small levers a few inches long were therefore introduced (power interlockings), which could be flicked over with a finger. These would be used in conjunction with the illuminated diagram , a diagrammatic representation of the track controlled by the tower, which had lamps that indicated the position of trains. Mechanical locking was still between the levers, but the bulk of the locking was becoming more and more electrical, using relay circuits rather than a mechanical means. Automatic interlockings, using only relay circuits to provide locking, appeared quite early. These were used at a few locations where traffic was light, and where no switches were involved. Examples were at-grade crossings of two railroads, or, in one case, a gauntlet track over a bridge. The earliest reported installation was in 1907, but this type of interlocking became more common in the 1920s. Signals were operated by track circuit occupancy only, and no control panel s were provided. In 1927, Sedgwick N. Wight of the General Railway Signal Company developed what was called Centralized Traffic Control (CTC). This essentially was (and still is) a system of remote control of a number of sequential interlockings, allowing one person (the dispatcher) to completely control train traffic in the equipped area. The old system of train orders and timetable operation was replaced by this system, which allowed commands to be sent to the train directly through the wayside signals. Train operation on wayside signal indications had been in operation in several areas, but the interlockings were still individually controlled. The dispatchers had to rely on the individual towermen either to route trains on their best judgment, or to act on the dispatcher's direction (usually verbal). Several significant features were included in this CTC system: remote control of interlockings, all-relay interlockings with switches, and no mechanical interlocking anywhere. In 1929, a power interlocking was installed with no locking on the levers at all, all the safety being provided by relay circuits. This was considered to be a revolutionary step at the time, but it was quickly realized that if there was no locking on the levers, then there was no longer any reason to have the levers. When using a lever frame, the towerman had to set each switch to the correct position by individual levers, and then pull a lever to change the signal. With the elimination of levers, a new technique was possiblea route-type panel. All the towerman had to do was to push two buttons. The first button was associated with the signal at the start of the move wanted; the second button, with a signal at the end of the move. The interlocking system then arranged for all the switches to be set the correct way without the towerman having to worry about them individually. This method of control (a GRS invention) is known as an Entrance/Exit (NX) route-setting system and is widely used throughout the world. As train speeds increased, the time the train operator had to see si.gnals decreased. It was recognized that a continuous display of the signal, in the cab of the locomotive, would be advantageous. The Interstate Commerce Commission had been proposing automatic train control as early as 191 1, and in 1922, ordered installation of train control on 49 railroads. Several railroads felt that the train-control devices available

7

1 · Railway Signaling

at the time were not satisfactory (many such systems used a wayside ramp that made physical contact with the train). Continuous cab signals were experimentally installed in 1923, and in 1925, the Atchison, Topeka & Santa Fe (AT &SF) made an installation without wayside signals. Cab signals were eventually installed over significant areas and are today a requirement on the Northeast Corridor between Washington, D.C., and Boston, Massachusetts. With this amount of information being held on the train, there is no real need to have physical signals anymore. On the high-speed lines in France and the Channel Tunnel, there are, in fact, no signals. The Docklands Light Railway in the United Kingdom (as an example) has no signals- and no train drivers either! Various systems taking the place of the towerman and the driver interact with the interlocking to ensure the completely safe operation of the line. A recent, and very significant, development took place in the early 1980s. The techniques used with relay interlockings had been built up over a period of at least 70 years . The ways that vital signal relays could fail were well known and the circuits could be designed, knowing these facts , to ensure that should a failure occur the whole system would remain safe. Electronic components fail in many different ways, so it is very difficult to guarantee safety. Electronics were used in applications usually limited to discrete components, where the range of possible failures was manageable. Microprocessors were in use m industry generally, but the range of fa ilure modes was unacceptable fo r the analyses available. Several different elements combined in 1978 to prompt the start of vital microprocessor development. Processors had been applied to nonvital app li cations. Discussions with peop le involved in microprocessors and familiar with rail road safety techniques revealed that very different methods of safety assurance would be requ ired than had been appl ied before. Explorations of these new methods, along with the experiences with nonvital applications, encouraged the deve lopment of vital applications. In 1983, the first test installation of a vital microprocessor interlocking system was turned on. The first full installation by GRS (General Railway Signal Company) was in 1986. US&S (Union Switch and Signal) had placed their first installation in service in 1985, although the interfaces all required relays. Today, entirely new approaches to si.gna ling are being developed. Communication Based Train Control (CBTC), known also by several other names, extends the application of microprocessors fo r safety and adds sophisticated communications systems to produce a signal system requiring very little equipment along the tracks. ln place of track circuits, the train determines its own location. This information is transmitted to central processors, which, in turn, tell each train how far it can go based on position of trains ahead and other resh·ictions. Each train then determines its own speed limit using both the information from the central processors and its own "knowledge" of the track speed limits, grades, and other factors. These systems may eventually replace a signifi cant portion of the equipment and systems described here. However, such changes will not occur overnight; many of the older systems will remain in service for many years, and will continue to be expanded and upgraded. The basics and principles will continue to be important. As an examp le, the principles of locking in a mechani cal interlocking as described in 1907 are embedded today in microprocessor interlocking systems.

8

Chapter 2

The Elements of a Signal System

HACKENSACK DRAW ERIE RAILROAD DIAGRAM OF TRACKS AND SIGNALS

ERECTED BY AND UNDER THE PATENT RI GHTS OF THE GENERAL RAILWAY SIGNAL

Co.

ROCHESTER N.Y. U.S.A.

A. J. R. Rowbotham K. D. G. Bisset 9

What Is a Signal System?

Introduction What Is a Signal System? One of the concepts remembered from school days is that the first step of any analysis is to define exactly what is being analyzed. For example, in ana lyzing an engine, one might define the engine itself as the system, and the inputs are fuel and air while the outputs are heat, mechanical work, and the exhaust. Auxiliary function s, such as the alternator and water p ump, could be considered "outside" the system being analyzed. It is traditionally assumed that a signa l system includes a large number of pieces of hardware and their interconnections. This hardware inc ludes signals, switch mach ines, track circuits, control relays (and now microprocessors), crossing flashers and gates, and ancillary devices. The ancillary devices vary depending on the railroad, and might include such devices as snowme lters (for switches), dragging equipment detectors, hotbox detectors, rockslide detectors, car retarders for yards, train stops and other enforcement devices, and many other devices, parts, and systems. Many of these items still require definition of the limits of the "signal" work. For example, a switch machine requires connection to the track. Typically, the th.row, point detector, and lock rods making these connections are the responsibility of the signal department; the gauge plates and the rods connecting the points together are the responsibility of the track forces. Similar divisions occur with other systems, req uiring the signal designer to interface w ith communications, track, and even occasionally structure personnel. Even though these may not be the responsibility of signal personnel, they affect the signal system and its design, so conscientious signal designers must have at least some knowledge of these areas since they affect their work. The signal designer must also be familiar with regu lations and standards affecting the work. Perhaps the most fami liar of these in the United States is the RS&I, the collection of U.S. government regulations whose short title is Rules, Standards and f nstructions for the Design, Installation and Maintenance of Railroad Signal Systems (U.S. Department of Transportation, Federal Railroad Administration, 49 CFR 236, Rules, Standards, And Instructions Governing The Installation, Inspection, Maintenance, And Repair Of Signa l And Train Control Systems, Devices, And App liances). State regulations govern many aspects of highway crossing warning systems; however, since the passage of the FRA's Part 234, the Federal Railroad Administration (FRA) now has significant jurisdiction over highway crossings. Each railroad has further standards of their own for all signal systems. The American Railway Engineering and Maintenance-of-Way Association (AREMA), now incorporating the former Association of American Railroads (AAR), Conununications and Signal Division publ ishes a manual ofrecommended practice for all signal systems, titled the AR.EMA Communications and Signals Manual of Recommended Practices. The title is often shortened to the AR.EMA Signal Manual. These items constitute only the tangible portion of signaling; several larger (and, in some ways, more significant) parts of signaling are seldom addressed in writing. Perhaps these might be lumped together under "personnel" issues; including the following: • • • • •

Operating practices Operating rules and their enforcement The phi losophy of the signal system Signal maintenance practices Signal design and testing practices

11

2 - THE ELEMENTS OF A SIGNAL SYSTEM

Consider also the history, both of signaling in general and of the installation under analysis. Though this history should be a separate book, some history has been inc luded here where it seems appropriate. Most of the designs and practices used in signaling have root in some past event, perhaps where someone discovered some deficiency and made a modification to solve it. Some practices are a result of tradition, practices developed when other methods were not available- " it's always been done this way." Perhaps the most easily recognized "historical" components of signaling are the results of accidents. The accident is well documented and remembered because of its sensational and h·agic nature, so the event prompting change is clear. This is not the case with most events causing changes in signal design. It has been enlightening to read old AAR Proceedings and Reports, and learn how some past problems were solved. This only covers a small portion of these solutions, but one gets a sense of the effort and study invested. Returning to the question, What is a signal system? a specific definition of a s ignal system will not be provided here. The boundaries of a signal system are not well defined; further, they are different on each project, and are moving as such systems become more complex. Be prepared to consider this question throughout each project.

The Railroad The visible parts of the railroad that need to be considered consist of the track, the signals, and the trains. The track is what the trains run on, and allowance must be made for the tracks to diverge and converge. These junctions are known as switches or turnouts and a device is needed to move the switch from one position to the other to set up the correct path. Th is device is known generically as a switch machine and is one of the elements that the signaling system has to contro l. These are discussed in more detail in Ch. 3, Track Work and Ch. 4, Switches. E lectrical circuits in the rails, track circuits, are used to determine whether a particular section of track is clear of traffic or not. Further description w ill be found in Ch. 7, Train Detection. Signals are placed alongside the track and give instructions to the train operators. Modern s ignals give their instructions by means of lights, easily visible by day or night. Thus, signals are also one of the elements that the signaling system has to control. Signals themselves are covered in Ch. 5, Wayside Signals. T he trains take many forms, and their operation affects (and is affected by) the signal system. Lt is the trains that the signal system must keep apart. The braking characteristics of the trains will affect the arrangement of the signals (naturally-the signal system controls the train by telling the engineer to stop, and there must be enough warning so that this is possible). If cab signals are included, there must be equipment onboard the train, as well as interfaces to the train systems (such as the brakes).

Signal Systems In the most genera l terms, the practice of s igna ling can be divided in perhaps four major categories. B lock signals keep trains apart on track without interlockings. They may take many forms, both in the type of signals and in the controls. The signals may be color light, searchlight, or any of the other types of wayside signals. T hey might also be cab signals, where the aspect is di splayed in the cab of the

12

Signal Systems

locomotive (or control car). Cab signals might be used in conjunction with wayside block signals, or may replace many of the wayside signals. The controls might be arranged for simple, single-direction operation where two or more tracks are available. In single-track areas, the most common block signal control is Absolute Permissive Block (APB). The name derives from the abso lute protection for opposing (head-on) train movements, w ith permissive (STOP AND PROCEED) protection for following movements. If adjacent interlockings are used to control entrance to a section of track (Centralized Traffic Control, or CTC, for example), the block signals will be intercormected with the interlockings and traffic circuits. As implied above, the second major category of signaling is interlocking. An interlocking is defined as: "An arrangement of signals and signal appliances ... so intercormected ... that their movements must succeed each other in proper sequence, train movements over all routes being governed by signal indication" (RS&I). Interlockings are used when there are conflicting train movements, as in the case of switches and crossings. Block signals separate trains on the same track, but interlockings separate trains where tracks cross or come together. The signals along the track (also known as wayside signals or.fixed signals) take the same form as the block signals, but are typically absolute, not permissive, signals (absolute signals at STOP may not be passed w ithout specific permission). In the United States, signals are provided at all track entrances to interlockings, to comply with the last part of the definition. Similar practices are followed in Canada. The third major category is highway-rail grade crossings. The design of warning devices for highway grade crossings and their controls is a significant portion of the overall signal design effort. Note that the term "warning" is used, not the old term "protection" because of the extreme difficulty (or even impossibility) of "protecting" against the acts of some motorists. The fourth category might best be described as "everything else." There are many e lements here that the signal designer must contend with, at least on occasion. Remote control (code) systems might be placed here. Hotbox detectors, retarder yards, rockslide fences, and a myriad of other detection systems can become part of the signal system, or at least interface with the signal system. Some types of train-to-wayside conununication systems, train identification systems, and other spec ialized equipment might come under the signal system, especially on a transit operation. A brief overview of the first three groupings discussed above is summari zed in the following diagram:

13

2 - THE ELEMENTS OF A SIGNAL SYSTEM

Signaling Roadmap

Control and f Verify Switch Position

Locate Trains

Control and Verify Signal Position

Lock Switches and Signals

Inform Motorists

Inform Other Trains Trains

Highway Crossing Warning

Block Signals Interlocking

Signaling Operations A few of the obvio us signaling elements have been mentioned above. It is very important to realize that the sign al system is not an end to itself; it is a part of a complete transportation system known as a rai lroad (o r railway). Because of this, there arc many interfaces w ith other parts of the railroad; for example, the signal designer must know about the track and interface with the track desig ners. A parti cularly important interface is with the train operator. The signals and controls are des igned to be sure that the tra in operator is given the correct aspects, but what do these aspects mean? The train operator (known also as the driver, motorman, and other terms) relies on the rulebook (operating rules and their enforcement) to describe the nam es and indications associated with each signal aspect. T he indicatio ns, in turn, describe the action to be foll owed. Further, the rulebook defines many other interfaces between the train operator (and indeed the operating department) and the signal system, inc luding action at a failed signa l, and the operation of various ty pes of switches. Thus, the rulebook is a very important part of the signa l system, and the signal system designer needs an understanding of the rulebook to define the system operatio n. On most rail properties, the signa l engineer is included in the ru lebook comm ittee.

14

Automatic Train Control

Other Operating Considerations Even though not directly connected with the signal system, a few other features of train operation may be of interest to the signal designer. Train operators must be very familiar with the physical characteristics of the portion of the railroad on which they are operating. Curves, grades, highway crossings, and signals are among the items that affect the operation of the train. If train operators should have to operate over unfamiliar territory (for example, due to an incident requiring a re-route), a person familiar with the territory (called a pilot) must accompany them. The air brake system for trains is complex, and there are books covering just the braking system. In sim ple terms, there is a supply of compressed air on the locomotives. In normal operation on a train, this air is fed (at around 100 psi) through hoses and pipes (the brake pipe) through the entire length of the train. On each car, the air is fed through a valve to fill reservoirs. To apply the brakes, the pressure in the brake pipe is reduced, and valves on each car allow air to flow from the service reservoi r to the brake cylinder. If it is desired to apply the brakes harder, the pressure in the brake pipe is reduced further. To release the brakes, the pressure in the brake pipe is increased, but it is not possible (with most present-day brakes) to partially release the brakes. A large, sudden reduction in the brake pipe pressure (which will occur if the cars separate) will apply the brakes in emergency. Most locomotives and control cabs are equipped with some form of a so-called "dead man control." The train operator must hold a lever or pedal down, or the brakes will be applied. Alternative forms require the train operator to move or touch and release the controls. In some cases, an audible alarm will sound before the system applies the brakes.

Highway-Rail Grade Crossings Where a road or highway crosses the track at the same e levation, it is known as a highway grade crossing, or often simply a grade crossing. There are a large number of varieties, some with flashing lights, cantilevers, and gates; some with flashing lights only; some with only a sign (often called a

crossbuck); and a few with no identification at all. W here traffic (both road and ra il) warrants, warning devices are commonly installed. In past years, these often were operated by a crossing watchman, but the expense led fairly quickly to automation. Today, active warning devices (fl ash ing lights, with or without gates and bells) are controlled either by track circuits, or by a motion detector or motion predictor. These latter devices measure the impedance of the track, and as the train approaches, the change in impedance is used to develop control of the warning system. It is very rare in America to find any signal for the train connected to the crossing system. Further description w ill be found in Ch. 12, Grade Crossings.

Automatic Train Control To improve safety, it is possible to pass information from the track to the train and give the train operators additional information to assist their task, particularly important as train speeds increase. Cab signals bring signal aspects into the locomotive cab so that the aspect is continuously visible. With a more advanced system, the cab signal aspect can be enforced onboard the train, and require the train operator to at least acknowledge any reduction in aspect. If the train operator does not respond within a short time, the train brakes wou ld be applied automatically. Continuous speed control is also possible with a cab signal system. Further description will be found in C h. 11, Cab Signals.

15

2 - THE ELEMENTS OF A SIGNAL SYSTEM

Hotbox Detectors It is necessary to monitor the bearings of axles of vehi cles at strategic points. Bearings can heat up to the point that the end of the axle burn s off, causing a sudden derailment. Hotbox detectors use infrared scanners that are mounted to the rails or adjacent to the track, and measure the journal bearing temperature of every passing axle. T he number of passing axles is counted, so if a "hotbox" (an overheated journal bearing box) is detected, a message is passed to the train indicating which ax le of the train is affected. Hotbox detectors are normally positioned on the approach to suitable facil ities for taking the faulty vehicle out of service.

Line Circuits A signal location, whether it is a single signal or a major interlocking, can only rarely exist in total isolation. There is almost always a connection to another location, usually to (at least) select the yellow or green signal aspect. Historically, these connections were often carried on wires supported by "telephone" poles along the right-of-way. Since these wires were generally not bundled in a cable, they were called open line wire. Signal circuits usually used insulated wire (communications circuits, such as telephone and telegraph, usually used bare wire) and the wires were subj ect to contacting each other (crosses) or the earth (grounds). To reduce problems associated with these connections, trees were kept clear of the pole line, and often both wires of line circuits were opened by two contacts on the same relay. T his is known as a double-break (or double-cut) circu it. Pole Jines have become very expensive to maintain, and today electronic track circuits such as Genrakode'" and Electro Code®transmit the required info rmation between locations through the rails, as part of the track circuit. Where many connections are required, the wires are assembled in a cable, which may be carried on poles above ground (aerial) or underground . The insulation between conductors is an important part of the safety of the signal system, and so is much more generous than what one would find in cables for general uses.

Safety The signaling system has to perform in a safe manner, but perhaps more importantly, if something goes wrong, it also has to fail in a safe manner. For instance, if certain things go wrong with the controls of a signal, the signal m ust immediately show a red STO P indication to the train operator. It is a nuisance for a signal to show red when it should be showi ng green but not dangerous. This type of failure is known as a safe or rightside failure. However, for a signal to be showing green when it should be showing red would be very dangerous indeed, an d this type of failure is known as an unsafe or wrongside failure, or sometimes as a false clear. These overall requirements are described as fail safe and are the basis of all signali ng systems. R ightside failures are uncommon but do occur, but wrongside fai lures are extremely rare and each would be the subject of a rigorous investigation. Safety is the overriding concern in signali ng. Consideration must always be given to odd conditions such as a wire break or power failm e. These must not cause a wrongside fai lure, but it is not always obvious how such conditions can affect the system operation.

16

Design Safety

The Safety of a Signal System The prime purpose of a signal system is "to facilitate the safe and efficient movement of trains ..." Safety and efficiency are not always compatible; for examp le, one train at a time on the railroad is probably safe (there wi ll certainly be no collisions with another train), but it does not make very efficient use of the track. A signal system does not necessarily improve operation; in fact, it will sometimes slow train operation. lt will, however, facilitate train movements in a safe manner, and this is the primary goal of a signal system. Safety is an extremely important consideration in the signal system, and one that has developed over the years into a number of facets. T his discussion wi ll not inc lude safety relating to working conditions and personal safety. The railroad can be a hazardous environment, and track safety is very important, but it wi ll not be discussed here. Even though system reliability is very important, it is not part of this discussion, either, except to point out that an unreliable system may be as bad as (or perhaps worse than) no system at al l. When the signal system fails , other methods may be required to move the trains, and these other methods usually will rely on people to provide the safety. Some of the documents and organizations referred to below are: • RS&I: U.S. Department of Transportation, Federal Railroad Administration, Code of Federal Regulations Title 49, Part 236 (49 CFR 236), Rules Standards and Instruction Governing the Installation, Inspection, Maintenance, and Repair of Signal and Train Control Systems, Devices, and App liances. • AREMA: American Rai lway Engineering and Maintenance-of-Way Association

Fundamental Safety The first step in providing a safe signal system (or, in fact, any system) is to define the fundamental principles. A few of these are defined in the RS&l; for example, opposing signals (signals which, if cleared, cou ld allow a head-on collision) should not clear at the same time. Others are in the AREMA Communications and Signals Manual of Recommended Practices. Still others may be defined by the railroad in the form of the contract documents, control lines, typical circuits, or route and aspect charts. There are additional fundamental principles that are poorly documented, or not documented at all. Many of these are related to operations that would seem to be very unlikely, but which must be dealt with in the signal system. Long experience will teach many of these principles. Experience w ith a railroad will teach that railroad's principles. These same principles may not be used or may be modified on another railroad . Some examp les of fundamental principles are: • Detector track circuit occupied w ill lock a switch. • A governing signal clear will lock a switch.

Design Safety Once the fundamental principles are understood, they must be translated into a safe design. Perhaps the first concept used here is the closed circuit principle in combination with vital relays or other vital devices .

17

2 - THE ELEMENTS OF A SIGNAL SYSTEM

construction sequence. Testing for a microprocessor interlocking system will vary in detail from that for a relay-based system, for example. It might not be practical to perform some testing in a factory when multiple racks or housings are shipped at different times. ln any case, the testing, testing, and more testing is a very important part of the safety of the signal system. Any testing must compare the system as built to some "standard," and the standard used varies depending on the type of test. The objective is to be sure that no errors are undetected or uncorrected, at any level. Testing usually starts at a very basic level. Are the components correct, and installed correctly? ls each item correctly identified? This type of testing verifies that the system agrees with the design details, and the testing compares the construction to the "detail" drawings such as rack layouts. Power buses are tested, to be sure that there are no crosses, grounds, or open circuits. Every wire is tested for continuity (buzzed), its name (nomenclature) checked, and the number of wires on each terminal is verified. This testing compares the actual wiring to the circuit drawings, to confirm that the wires are installed correctly. So-called " breakdown" testing checks that each circuit is completely wired in accordance with the circuit design. Each contact in a circuit is opened and closed to observe that the relay (or other connected device) is energized and de-energized in accordance with the circu its as drawn. The buzz and breakdown tests are sometimes called greening-off, because as each wire is verified, it is often marked with a green pencil, and other colors may be used to confirm the other items to be checked. At the completion of this level of testing, the constructed system has been verified against the drawings. A few design principles might have been verified, but the entire loop back to the fundamental safety logic has not been closed. To complete the testing, the operation of the overall system must be confirmed. This is the operational test, and is often performed in stages. The first stage wou ld be performed when the system, or a s ign ificant po1iion such as a comp lete interlocking, is assembled ready for connection to the field equipment. This is sometimes known as preoperational testing, and may be performed in the factory or in the field. The field conditions arc usually simulated (track circuits, switch machines, and other such connections) and the complete system operation verified. This testing compares the system operation with the fundamental safety logic with which the design started. Ideally, this testing will be performed without reference to the circuit design, except if (when?) discrepancies are found. The final stage is the operational or in-service test, performed when the field equipment has been connected and tested. It is frequently performed immediately before placing the system in service, and requires close coordination with the railroad because usually the track area involved must be taken out of service, or severe train operation constraints must be in place. This last testing is often performed by the railroad, although transit properties often witness the testing performed by the contractor. The above description is simplified and a broad overview, but the principles must be followed. Each installation needs test procedures to be sure that nothing is missed, especially when the testing is being performed in a limited time, such as placing the system in service (also known as a cut-over). The steps may not follow exactly the sequence above-for example, field connections must undergo breakdown testing, and this wi ll usually be done very shortly (a month to a day) before the cut-over, if not during the cut-over. Simulations may be used at various levels, but the final actual system must still be tested against the design principles before allowing operation.

20

Design Safety

The Safety of a Signal System The prime purpose of a signal system is "to facilitate the safe and efficient movement of trains ..."Safety and efficiency are not always compatible; for example, one train at a time on the railroad is probably safe (there wi ll ce11ainly be no collisions with another train), but it does not make very efficient use of the track. A signal system does not necessarily improve operation; in fact, it will sometimes slow train operation. It wi ll , however, facilitate train movements in a safe manner, and this is the primary goal of a signal system. Safety is an extremely important consideration in the signal system, and one that has developed over the years into a number of facets. This discussion will not include safety relating to working conditions and personal safety. The railroad can be a hazardous environment, and track safety is very important, but it will not be discussed here. Even though system reliability is very important, it is not part of this discussion, either, except to point out that an unreliable system may be as bad as (or perhaps worse than) no system at all. When the signa l system fa ils, other methods may be required to move the trains, and these other methods usually will rely on people to provide the safety. Some of the documents and organizations referred to below are: • RS&I: U.S. Department of Transportation, Federal Railroad Administration, Code of Federal Regulations Title 49, Part 236 (49 CFR 236), Rules Standards and Instruction Governing the Installation, Inspection, Maintenance, and Repair of Signal and Train Control Systems, Devices, and Appliances. • AREMA: American Railway Engineering and Mai ntenance-of-Way Association

Fundamental Safety The first step in providing a safe signal system (or, in fact, any system) is to define the fundamental princ iples. A few of these are defined in the RS&I; for example, opposing signals (sign als which, if cleared, could allow a head-on coll ision) should not clear at the same time. Others are in the AREMA Communications and Signals Manual of Recommended Practices. Still others may be defined by the railroad in the form of the contract documents, control lines, typical circuits, or route and aspect charts. There are additional fundamental princip les that are poorly documented, or not documented at a ll. Many of these are related to operations that wou ld seem to be very unlikely, but which must be dealt w ith in the signa l system. Long experience wi ll teach many of these principles. Experience with a railroad will teach that railroad's principles. These same principles may not be used or may be modified on another ra ilroad. Some examples of fundamental principles are: • Detector track circuit occupied w ill lock a switch. • A governing signal clear wi ll lock a switch.

Design Safety Once the fundamental principles are understood, they must be translated into a safe des ign. Perhaps the first concept used here is the closed c ircuit principle in combination with vital relays or other vital devices.

17

2 - THE ELEMENTS OF A SIGNAL SYSTEM

Several terms need to be defined to allow discussion of these ideas. • Vital circuit: "Any circuit the function of which affects the safety of train operation." (AREMA Signal Manual, Part 1.1.1) • Vital: Affecting the safety of train operation (by extension of above). • Vital relay: A relay designed with established fail ure modes that can be used, in combination with design practices (such as the closed circuit principle), in vital circuits. This applies to other devices as well. Note that a "vital relay" can be applied in an unsafe manner. • Fail safe: "A term used to designate a railway signa ling design principle, the objective of which is to eliminate the hazardous effects of a fa ilure of a component or system." (AREMA) • Fail safe: "A characteristic of a system whi ch ensures that a fault or malfunct ion of any e lement affecting safety will cause the system to revert to a state that is known to be safe; a lternatively, a system characteristic which ensures that any fault or malfunction will not result in an unsafe condition." (Automatic Train Control in Rail Rapid Transit) • Principle, closed circuit: The principle of circuit design where a normally energized electric circuit which, on being interrupted or deenergized, will cause the controlled function to assume its most restrictive condition. (RS&I) Circuits and subsystems can be described as vital or nonvital. Usually, control panels are examples of nonvital subsystems, but a signal contro l circuit is considered vital. Nonvital circuits and systems do not need to fo ll ow the same practices as vital circui ts, nor do they need to use vita l dev ices. Fai lures in a nonvital system may cause undesired or inconvenient results (for example, setting a signal to red in front of a train, or requesting an incorrect signal to clear), but such results will not be unsafe. T he vital system that is connected to the nonvital system must be des igned to prevent any nonvital fa ilures from causing unsafe results. As this implies, there w ill be interfaces between v ital and nonvital systems. These interfaces blur the demarcation between the two systems. Nonvital contacts may be controlling vital relays, and vice versa. The latter seldom requires specia l consideration; the former requires great care, especially if both vital and nonvital contacts are present in the same circuit. The closed circuit design principle provides some gu idance, but the designer must interpret the application to determine which is the most restrictive cond ition. As an example, the statement above reads "Detector track circuit occupied will lock a switch." In implementing this statement, it perhaps would be more precise to say "Detector track circuit unoccupied will allow a switch to unlock." This emphasizes that the lock relay (or function) should be norma lly energized, and can be energized only when the detector track relays are energized. It also suggests that other conditions may (in fact, do) lock the switch. T he design must take into account the hardware being used. The closed circuit principle is based on the use of North American vital relays, which have known and accepted failure characteri stics. The (usually unwritten) assumption is that the front contacts of a vital relay will always open when the coil is de-energized. (A more complete and accurate statement is that the front contacts wi ll not remai n closed longer than intended when the current in the coil is reduced below the drop-away value specified. This recognizes s low-drop relays.) If other devices are used, their failure modes must be understood so that they may be properly applied and still maintain the required safety. One method commonly applied is to cycle check the device. An example of thi s approach is a mechanical train stop, which drives to the clear position with a motor and uses a spring to return to the tripping position. The spring might break, and the protection needed from the train stop would be lost. The circuits are arranged so that the assoc iated signal ca nnot clear unless the train stop mechanism is detected in the tripping position. Once the check is

18

Construction Safety

performed, the signal contro l relay can pick and drive the trip to the clear position. A stick contact on the control relay bypasses the check contact. Thus, the train stop must return to the tripping position before it can re-clear; in other words, it must cyc le between the tripp ing and clear positions. Here, the cycle check is performed each time the signal clears, or it may be said that the cycle check is based on operati on. Depending on the device, cycle checking can also be based on time, where the check might be daily, hourly, every second, or even more frequently. This approach is very common in electronic devices. Other methods may be required, particularly in electronic equipment, but are beyond the scope of this brief discussion. Another exam ple of a design consideration is the provision of slow-drop features on vita l relays. It may be necessary to provide de lays in circuit operation, and this may be accomplished in several ways. Two examples are a slow-drop relay, or a capacitor "snub" (frequently, a slow-drop relay is said to be snubbed or slugged) . If part of the coil of a relay is replaced by large copper washers or a solid cylinder of copper, the currents induced in the copper when the coil is de-energized will keep the annature in the energized position for a time (as much as a few seconds). Another way to make a relay slow drop is to add a capacitor in parallel w ith the coil. A resistor is usually placed in series with the capacitor, to limit the inrush when the circuit is energized, and to limit the current if the capacitor should sh01t. It is not hard to obtain drop-away times from a few seconds to as much as several minutes by selecting the va lue of the capacitors. The designer must consider what happens if the connection to the capacitor should open, or if the capacitor should fai l. If the time delay is critical to the safety of the operation, it probably would be better to use a relay w ith a slow-drop copper slug, because there is much less possibility of that device fai ling than of a capacitor circuit opening . The design itself is documented in the draw ings prepared for the signal system. The drawings are an important part of the safety of the signal system, because they not only describe the design but also show detai ls that define some aspects of the construction. The term drawings in this discussion includes the software li stings for microprocessor-based systems. Once the drawings are complete, the design must be checked to be sure that it complies with both the fundamental safety requirements and with the design safety requirements. A diffi cult part of the checking process is assuring that nothing was left out- it is relatively easy to see when a design is incorrect but harder to identi fy a missing design. Checking must include checks of the details, to identify termination points, for exampl e.

Construction Safety O nce the system has been designed and checked, it has to be built. The safety of the system is further addressed in this step. The techniques used to wire equipment is different from ordinary electrical wiring, in an effort to provide a safe (and reliable) system. Wire insulation is more rugged than typical electrical installations. Ring terminals are commonly used so that wires do not accidentally become disconnected. Term ina l posts are large and have large spacing, to minimize the possibility of accidental contact between w ires . These and other construction techniques minimize the possibility of unintentional connections between wires. Once the wiring is complete, the safety of the system is again verified by several layers of checking. At this point, the checking is performed by testing the system. Mu ltiple testing steps are used, in sequence, to assure that the wiring complies with the design, and that the system operates as intended . T he detai ls of the steps may vary depending on the company standards, on the equipment involved, and on the

19

2 - THE ELEMENTS OF A SIGNAL SYSTEM

construction sequence. Testing for a microprocessor interlocking system will vary in detail from that for a relay-based system, for example. It might not be practical to perform some testing in a factory when mu ltip le racks or housings are shipped at different times. In any case, the testing, testing, and more testing is a very important part of the safety of the signal system. Any testing must compare the system as built to some "standard," and the standard used varies depending on the type of test. The objective is to be sure that no errors are undetected or uncorrected, at any level. Testing usually starts at a very basic leve l. Are the components correct, and installed correctly? Is each item correctly identified? This type of testing verifies that the system agrees with the design details, and the testing compares the construction to the "detail" drawings such as rack layouts. Power buses are tested, to be sure that there are no crosses, grounds, or open circuits. Every wire is tested for continuity (buzzed), its name (nomenclature) checked, and the number of wires on each terminal is verified. This testing compares the actual wiring to the circuit drawings, to confirm that the wires are installed correctly. So-cal led "breakdown" testing checks that each circuit is completely wired in accordance with the circuit design. Each contact in a circuit is opened and closed to observe that the relay (or other connected device) is energized and de-energized in accordance with the circuits as drawn. The buzz and breakdown tests are sometimes called greening-off, because as each wire is verified, it is often marked with a green penci l, and other colors may be used to confirm the other items to be checked. At the completion of this level of testing, the constructed system has been verified against the drawings. A few design principles might have been verified, but the entire loop back to the fundamental safety logic has not been closed. To complete the testing, the operation of the overall system must be confirmed. This is the operational test, and is often performed in stages. The first stage wou ld be performed when the system, or a significant portion such as a complete interlocking, is assembled ready for connection to the field equipment. This is sometimes known as preoperationa! testing, and may be performed in the factory or in the field. The field conditions are usually simulated (track circuits, switch machines, and other such connections) and the complete system operation verified. This testing compares the system operation with the fundamental safety logic with which the design started. Ideally, this testing will be performed without reference to the circuit design, except if (when?) discrepancies are found. The final stage is the operational or in-service test, performed when the field equipment has been connected and tested. It is frequently performed immediately before placing the system in service, and requires close coordination with the railroad because usually the track area involved must be taken out of service, or severe train operation constraints must be in place. This last testing is often performed by the railroad, although transit properties often witness the testing performed by the contractor. The above description is simplified and a broad overview, but the principles must be followed. Each installation needs test procedures to be sure that nothing is mi ssed, especially when the testing is being performed in a limited time, such as placing the system in service (also known as a cut-over). The steps may not follow exactly the sequence above- for examp le, field connections must undergo breakdown testing, and this will usua lly be done very shortly (a month to a day) before the cut-over, if not during the cut-over. Simulations may be used at various levels, but the final actual system must still be tested against the design principles before allowing operation.

20

Ongoing Safety

Ongoing Safety Just plac ing the signal system in service does not end the concern for the safety of the system. rt is important to maintain the system in good order throughout its life. The RS&I, AREMA, and manufacturers' manuals all provide requirements and recommendations for continued testing of signal systems and equipment. Some examples of the items required to be tested by the RS&I include relays, locking, grounds, and cables . Relays must be tested periodically (at present every 4 years for most types- Section 236. 106) to be sure that they are within manufacturers' requirements. (Section 236. 106 does not indicate what is to be tested; see also Section 236. 101.) The usual relay test measures at least the pick-up and drop-away va lues, and has traditionally used a physically large variable resistor with a sliding contact to adjust the voltage and current. Because of this, the term often used is slide test, or sliding the relay. The various kinds of locking have to be tested every 2 years (at present), as described in Sections 236.376 through 236.381 . U ngrounded circuits must be kept free of grounds, according to Section 236.2, and must be tested every 3 months according to Section 236. l 07 (as of 2007). Cable insulation must be tested every I 0 years, unless resi stance is less than 500,000 ohms, when testing must be annually until repaired. Insulation tests used (and still use) an insu lation tester made by (in 2007) Megger Group Limited, and the tests are sti ll ca lled meggering, even though the name is a trademark. These are just a very few of the periodic tests required. It is important to understand that the signal system must be checked and tested to be sure that it is designed and maintained correctly throughout its life . A short introd uctory discussion cannot do more than hint at the importance of the safety considerations in a s ignal system. In some places in this work, safety wi ll be mentioned again as it applies to spec i fie topics. In other places, even though it may not be mentioned expl icitly, safety's importance w ill be apparent. Some topics might not seem to deal with safety at all at first glance, but one may well find safety's influence hidden on further study.

21

Chapter 3

Track Work

A. J. R. Rowbotham K D. G. Bisset 23

Rails

Introduction The railroad track provides a path on which trains run, and which directs trains in the appropriate direction at junctions. Basically the track consists of steel rails on which the trains run, supported on ties that hold the rails in position. The ties are, in turn, held in position by ballast.

Rails Very early rails came in several designs, but by the 1860s, the T-rail design became standard (shown below).

Rail is available in different weights, the heavier the traffic, the more substantial the rail section needed. Rail size in North America is expressed in pounds (lbs) per yard. Today, a light weight rail might be around 90 pounds per yard. The smallest generally used and readily available rail size is 115 pounds, but heavy traffic main lines might use 132-pound rail. AREMA (American Railway Engineering and Maintenance-of-Way Association) maintains standards for several different sizes of rail. In much of the rest of the world, rail standards are maintained by the UIC, which stands for Union Internationale des Chemins de Fer (i. e., International Union of Railways). A very common T-rail in use all over Europe is known as UIC 54, and the figure "54" is the mass in kg per meter (kilogram per meter). This section is almost identical to the British Standard BS 11 OA rail, the figure "1 1O" being the weight in lb per yard. European and British rails are a somewhat different shape from that used in North America.

25

3 - Track Work

Ballast The ties are held in place by ballast, which consists of stone chips. This needs to be packed around and under the ties to hold them firmly, but resiliently, in place. Today tamping machines, designed to pack the ballast between and under the ties, make track maintenance much less labor intensive. Care must be taken with these massive machines to ensure that any signal cables in the vicinity of the rails are not damaged.

Photo 2. Rail. tic plates, spikes, tics, and ballast

28

Rails

Introduction The railroad track provides a path on which trains run, and which directs trains in the appropriate direction at junctions. Basically the track consists of steel rails on which the trains run, supported on ties that hold the rails in position. The ties are, in turn, held in pos ition by ballast.

Rails Very early rails came in several designs, but by the 1860s, the T-rail design became standard (shown below) .

Rail is available in different weights, the heavier the traffic, the more substanti al the rail section needed. Rail size in North America is expressed in pounds (lbs) per yard. Today, a light weight rail might be around 90 pounds per yard. The smallest generally used and readily available rail size is 115 pounds, but heavy traffic main lines might use 132-pound rai l. AREMA (American Railway Engineering and Ma intenance-of-Way Association) maintains standards for several different sizes of rai l. In much of the rest of the world, rail standards are maintained by the UlC, which stands for Union Internationale des Chemins de Fer (i.e. , International Union of Railways). A very common T-rail in use all over Europe is known as UIC 54, and the figure " 54" is the mass in kg per meter (kilogram per meter). This section is almost identical to the British Standard BS 11 OA rai l, the figure " 110" being the weight in lb per yard . Ew·opean and British rails are a somewhat different shape from that used in North America.

25

3 · Track Work

Ties The rails are held in place the correct distance apart by ties, which are arranged transversely under the rails . Even today most ties are cut out of wood, but reinforced concrete ties have a substantially longer life. Steel ties are also encountered but are quite uncommon. Rail is supported on tie plates, and the rails are held in place by cut spikes.

An alternative to the spikes, and now very widely used, is the Pandrol clip, a piece of steel rod (bent into an almost indescribable shape) that acts as a spring and firmly holds the rail in place.

Other fo rms of track support include concrete ties, concrete blocks under each rail held the correct distance apart by metal bars, and paved track. Paved track consists of a continuous concrete slab (laid by a slip-form paving machine) to which the rai ls are fixed. It has the big advantage over conventional track in that it does not need as much vertical height. It is therefore particularly used in tunnels where the line is being electrified. It is very expensive, however, so its use is restricted. Yet another technique was used in the Channel Tunnel, which has probably the heaviest traffic of any railway. A flat floor of concrete was laid in the bottom of the tunnel. The rails were then brought in and fitted with concrete blocks at regular intervals. The rails were placed on the concrete base and carefully aligned and leveled. Concrete was then poured in to hold the track in place. If this was litera lly done, the track system would be a solid mass and crack up in a very short time. This problem was so lved by fitting a rubber boot around each concrete block before the rails were brought into the tunnel. These boots

26

Ties

provide sufficient resilience when the track is concreted in to prevent the breakup under traffic cond itions. Various designs of concrete-based track are often known as direct .fixation (sometimes abbrev iated DF) track, because the track is fastened directly to the concrete structure.

Photo I. One type of direct fixation track

27

3 - Track Work

Ballast The ties are held in place by ballast, which consists of stone chips. This needs to be packed around and under the ties to hold them firmly, but resiliently, in place. Today tamping machines, designed to pack the ballast between and under the ties, make track maintenance much less labor intensive. Care must be taken with these massive machines to ensure that any signal cables in the vicinity of the rails are not damaged.

Photo 2. Rail , tie plates, spikes, ti es, and ballast

28

Gauge

Joints Rails are manufactured in fairly short lengths and need to be joined together by joint bars to form continuous track. Joint bars consist of metal plates bolted through on each side of the rail.

)

I I.

~ I

0

0

0 .. ..

0 j

Joint bars need a lot of maintenance- the bo lts must be tight enough to hold the rails together firmly, but not too tight to prevent the longitudinal movement caused by the expansion and contraction of the rails resul ting from temperature change. In order to reduce mai ntenance, the track engineer likes to we ld the rails into long lengths, forming jointless (or welded) track, and eliminate the joint bars (another name for the jointless track is continuous-welded rail, or CWR). When the track is first laid, great care is taken to control the stresses in the ra il. The rail temperature is closely monitored, and the rail heated or stretched when the rail temperature is low. Rail anchors are applied to prevent the rail from sliding over the ties, and to maintain the correct position. Then, as the temperature varies, the rail is restrained from expanding and contracting by the rail anchors bearing on the ties, which, in turn, bear on the ballast. Thus, at normal temperatures, the rails will actually be in tension. Only when the temperature is high will the rail be in compression. In extremely hot weather, this compression may tend to distort the track, and jointless track needs to be laid very carefully with extra ballast to maintain stability. However, once laid, it lasts a very long time with minimum maintenance. American practice is to field-weld CWR strings together, producing truly continuous rails with no joints (except as described below). For track circuit purposes, insulated joints arc needed by the signal engineer. In jointed track, insul ation is incorporated in special j oint bars, and an insulating plate of the appropriate cross-sectional area is fitted between the rail ends to prevent any contact in hot weather. For insulated joints in jointless track, the most reliable way of making them is to take two short lengths of rail and make a glued (and insulated) joint between them under carefully controlled factory conditions. The joint is then taken out and welded into the track where required. This piece of rai l, about 20 feet long, is known as a plug.

Gauge T he distance between the rail heads is known as the track gauge. In North America, the gauge is typica lly 4 ft 8!1 in. and is known as standard gauge. 1t is c laimed that this strange measurement is a result of the distance between the wheels of Roman chariots. In metric measurements, this gauge is 1,435 mm.

29

3 - Track Work

There are a few exceptions to standard gauge, almost all in transit. 5 ft 2!4 in. gauge is used in Philadelphia, Pennsylvania, on some lines; 5 ft 2 ~ in. gauge is found in P ittsburgh, Pennsylvania, and in New Orleans, Louisiana; 5 ft 6 in. is used in San Francisco (BART), California; and 4 ft l O~ in. is used in Toronto, Canada. A few freight lines in Mexico are 3 ft gauge. The big problems, however, occur in countries such as Australia and India where there is a considerable mixture of gauges. In Australia, when the rai lways were bu ilt, no overall railway body existed, so each state acted independently with New South Wales having standard gauge, Western Austral ia, 3 ft 6 in. gauge (a lthough the extremely long line to the other States is standard gauge) and South Australia/Victoria, 5 ft 3 in. gauge. Slowly a standard gauge line is being built to join the states together to allow through-running . This involves sections of dual gauge track, a system with three rai ls. On one side of the track is one rai l, the common rail, while on the other side there are two rails, each at the appropriate distance from the conunon rail for the two gauges required. India is another problem country with a mixture of broad gauge and narrow gauge lines, not separated by their location in the country. Again a huge exercise is being carried out, converting a number of narrow gauge lines into broad gauge lines (which is a difficult and very expensive exercise as more space is needed). A very conunon gauge in mountainous areas such as Switzerland is meter gauge, and there is a vast network of strategically important main-line meter gauge railways in the alpine area. In winter these lines form virtually the on ly means of traveling. The Imperial equivalent of meter gauge is the 3 ft 6 in. gauge mentioned earlier. All the rai lways in southern Africa are built to this gauge, where it is known as the "Cape Gauge ."

Rail-Wheel Interface Most people believe that it is the flanges on the wheels that keep the wheels on the rails. This is only true on very sharp curves, when the flanges actually rub along the rails (usually audibly). If the flanges were in contact with the rail continuously then the wear would be intolerable and impractical. The drawing below, which is not to sca le, shows the interface between the rail and the wheel tread.

Note that the wheel tread is not parallel with the top of the rail head (although the slope is del iberately exaggerated in the drawing). Considering the case where the axle is tending to move to the left, the left-hand wheel will become a larger diameter at the point of contact, but the right-hand wheel will become a smaller diameter at the point of contact. As both wheels are attached to the same axle, they must

30

Track Symbols and Nomenclature

be making the same number of revolutions per minute. This difference in effective diameter w il I cause the axle to move back to the sih1ation where both wheels have the same effective diameter. A lot of research into the rail-wheel interface has resulted in the optimum profile shapes, resulting in very straight running with a minimum of hunting backward and forward and significantly reducing rail and flange wear.

Photo 3. The rail-wheel interface

Track Symbols and Nomenclature On track plans, it is not always necessary to show the details of both rails, so often a single line depicts a section of track. A single track is thus represented by one line on the drawing; a double track is represented by two lines, one for each direction. In North America, the railways usually "drive on the right" to match the roads. There are exceptions, though; for examp le, the Union Pacific has some lines (ex-Chicago & North Western trackage) w ith " left-hand running." Of course, with revers ible signaling, a track may be easily used in either direction . Every track must have a unique name so that it can be referred to in instructions or any other paperwork. In single-track areas, the track is known usually as the main track, and there will usually be sidings where trains can pass each other. In multiple-track areas, tracks are frequently numbered; on occasion, they may be designated by the timetable direction as well. Generally, a rai lroad will designate direction by the predominant orientation of the railroad. For example, the Milwaukee Road (which no longer exists) operated from Chicago, Illinois, to Seattle, Washington, and Portland, Oregon. Directions along the railroad were designated East or West, even when the track curved in different directions. A train moving north from Chicago to Milwaukee was, in the timetable, traveling west. In extreme examples, a train could be movi ng physically due east, but according to the timetable, it was traveling west.

31

Simple Switch

Introduction Switch and turnout are the terms u sed to describe the arrangement ofrails that allow junctions to be made in the track. Turnout is the term used by the track engineer, but others, often including the signa l engineer, frequently use the term switch. Strictly speaking, this use is not exactly proper, but it is qu ite common , and will be used throughout this chapter.

Switches and Crossings Simple Switch The formation of rails (looking from above) to make a switch is shown below. r

oy

Toe, Points \

stock Rail

-

N0 1 R0 d ·

'Guard Rail ' Closure Rails

Frog

\ _ Stock Rail

Simple switch

The outside rails, which are fixed, are the stock rails. The two inside rails, which can move from side to side at one end, are the points. The assembly of the points and stock rails is properly called a switch, but, for convenience and to fo llow common practice, this term will be used for the entire assembly. The end that can move is known as the toe of the points, and the two points are held the correct distance apart by a head rod or No. 1 rod. The location of the moving ends of the points is the point of switch, abbreviated PS, and this is frequently the position used to locate the switch on the plans. The area where one rai l crosses the other is known as thefi-og. As a wheel rolls over the frog, there is a sma ll gap in the rail where the flange of a wheel on the " other" route must pass. As there is a very slight possibility of a derailment at the instant the wheel passes this gap, guard rails are provided. These are positioned so that the wheel flanges pass between the running rail and the guard rail. The back of the wheel bears against the guard rail , which keeps the wheels on the correct path. With the points positioned as shown above, trains may pass along the straight track in either direction. If the two points were to be moved so that the upper point (in the diagram) was touching the straight stock rail (while the lower point was therefore not touching the curved stock rail), then trains may pass along the curved track in either direction. It is important to w1derstand which route is set by the positioning of the points. When looking from the point end of the swi tch toward the frog, if the right-hand point is against the stock rail (right point closed), the turnout is lined for the left-hand route, and vice versa. Switches are further classif ied (by the track department) as being right hand or left hand. Standing at the fac ing (point) end of the switch, if the curv ing track curves to the right, the switch is right hand and if the curve is to the left, the switch is left hand. Usually, this distinction is not critica l to the signal department, but it does help to know the terms.

35

4 - SWITCHES

Symbols and Nomenclature The words normal and reverse that are used to describe the way a switch is lying (or the switch's position) were introduced !Jlany years ago, and today are rather meaningless. When the decision has been made at the design stage about which position is to be termed normal and which is to be termed reverse, it is recorded on the track plan. On the track plan, the normal lie of the switch is shown by a triangle against the track. T he point of the triangle indicates the normal route. It is not possible when stand ing on the track to determine positively which is normal and which is reverse, unless (as on a few ra ilroads) letters are fastened to the tie to show which point must be closed for the normal position. Remember also that in this context, normal does not mean usual; nor does it mean straight. Track plans can be either single line (where one line represents a track), or double line (where each line represents a rail and two lines are required to show the track). The symbol s recommended by AREMA (American Railway Engineering and Maintenance-of-Way Association; the former Association of American Railroads' Communication and Signal Section is now part of AREMA) are appl icable to both types of track plan, and both will be used here. Be aware that other symbols have been used over the years, and the standard symbols used on a particular rai lroad may differ from the current AREMA sym bols.

If the switch was shown on the track plan as in (i) below, then the switch would need to be lying normal for moves between A and B, and would need to be reverse for moves between A and C. However, if the same sw itch happens to have been shown on the track plan as in (ii), then normal setting would be for moves between A and C, and reverse setting for moves between A and B.

'_______ ====== :

A -----~------ :

A _____

A

A

(ii)

(i)

A move approaching the switch where there is a possibil ity of a choice of route (such as a move from A to B or C above) is said to be a/acing move. A move in the opposite direction where there is no choice of route (from B to A, or from C to A, above) is said to be a trailing move . Normal and reverse refer to the di rection the points are set, while facing and trailing describe the direction of traffic. These are two independent sets of parameters; there is no correlation between the two. In the figure above, the parameters may be summarized in the following table:

Move

(i) Description

(ii) Description

A to B

Facing, normal

Facing, reverse

A to C

Facing, reverse

Facing, normal

B to A

Trailing, normal

Trai ling, reverse

C to A

Trai ling, reverse

Trailing, normal

36

Crossing

Crossovers A common formation of switches is a connection between the two parallel tracks of a double track. T his combination of two switches is known as a crossover and usually both switches are worked together under one contro l. Usually, the norma l position of the two switches would allow straight-through moves on both tracks, with both being reverse for a move between the two tracks (crossing over).

z : ~ Right Hand or Trailing

'4

..

:de Left Hand or Facing

'4

..

~ Scissors or Diamond

A crossover where a train could cross directly from one track to the other is a facing or left-hand crossover. The mirror image, the trailing crossover (or right-hand crossover) requires the train to back up to cross over from one track to the other (assuming, in both cases, right-hand running). If both types were to be needed in the same vicinity, they would be called a universal crossover (not shown here). If there is not enough linear space avail able, the two types may be superimposed on top of each other, forming a scissors crossover or diamond crossover. This introduces a new feature of track work- two tracks crossing over each other at the same level.

Crossing The formation of rails to make up a crossing (or diamond crossing, reflecting the shape the rail s make in the center, or simply diamond) is shown in the fo llowing drawing. There is no usable connection between the two tracks involved.

At each end there is a frog similar to that found on a switch, and in the middle are two obtuse frogs. These frogs also have to have gaps to allow the wheel flanges through and have protective guard rails, forming a "K" shape. Hence, these frogs are sometimes known as K-frogs.

37

4 - SWITCHES

On a track plan, the two tracks just cross each other at an angle.

When the angle of the crossing becomes very narrow, the gaps at the K -frogs become unacceptable. The solution is to work the two obtuse frogs as two sets of points- the movable-point frog (MPF). Movable-Point Frogs In the diagram, the two sets of points are shown with their rods.

Movable-point frog one

The left-hand set of points are positioned so the lower point (on the drawing) is touching the lower obtuse angle, whi le the right-hand points are positioned touching the upper obtuse angle- which makes a path from the upper left track to and from the lower right track, with no gaps at the obtuse angles. For a train to move along the other diagonal path, both sets of points have to be moved, in opposite directions, to their other position. On a track plan, the usual rules apply with the normal position of the switch diamond shown by the point of the triangle. In the diagram, the normal position is for moves from upper left to lower right (or lower right to upper left), and the reverse is for moves on the other track.

Slips

If the angle of the crossing is small enough, it is possible to add rails between the outer crossings to allow movement around one of the corners. Such an arrangement is known as a single slip (you can "slip" around one of the corners).

38

Slips

In the diagram, it is possible to make a move between the lower two ends. It would be equally possible to allow movements between the upper two ends instead with a mirror image of this arrangement.

The normal position shown on these symbols is for crossing moves on either straight track; that is, from upper left to lower right (or vice versa), or from upper right to lower left (or vice versa). If both moves are required, the track work becomes complex and the combination is known as a double slip.

The symbols for the double slip are:

The route available in the normal position as shown on these drawings is from lower left to upper right. It is poss ible to have a double slip with movable-point frogs , but thi s combination would be expensive. It

is used in complex, compact track areas such as the approach to large passenger stations.

39

4 - SWITCHES

Derails It is sometimes necessary to deliberately derail a train to protect a second train from a possible collision, or to prevent a train from going through an open drawbridge. Two different methods, split point derail and lifting block derail are used in different circumstances.

Split Point Derail When speeds may be relatively high (perhaps 25 mph or more), or it is important that the train be derailed with absolute certainty, the split point derail is used. It is similar to a switch, but with only one point and no frog.

In the diagram, a train moving from right to left will be derailed when the point is open, as shown (th is position is usually defined as the normal position). The point would usually be controlled as part of an interlocking, and the signals governing movements over the derail would be locked at danger until the derail had been moved to the position to allow the train to pass. The split point derail will very reliably derai l the train, but it requires a break in the running rail.

Lifting Block Derail When speeds are low, a simpler dera il will be quite effective. The lifting block derai l consists of a casting that rests on the head of the rail in the derailing position. The casting is shaped to lift the wheel and guide the wheel flange over the top of the rail to the outside of the track. To allow trains to pass without derailing, the casting is arranged to either slide sideways off the rail, or to rotate away from the rail. Thi s type of derail is frequently used near the switch connecting a short industrial siding to the main line so that cars left on the siding w ill not accidentally roll onto the main track. In this application, the derail might be operated by the switch stand (which is usually manual) and would be pipe-connected so that throwing the switch for the siding w ill move the derail to the CLEA R position. Alternately, the derail could be manually operated directly and cou ld operate contacts that would put main-line block signa ls to STOP if the derail is not in the derailing position. Again, the dera iling position is usually chosen as the normal position, and the symbols below show this position. If the normal position were in the nonderailing position, the triangle symbol would be shown a small distance away from the rail.

•

• 40

Switch Machines

Switch Machines The position of the points is controlled by a switch machine, which drives the points to the correct alignment. The switch machine is fixed to extra long ties, usually referred to as the headblock, or No. I and No. 2 ties, adjacent to the point of the switch.

SWITCH MACHINE

~

~

1

1

n

~~ ~

~ ~

~

I

~

I

D LJ D D

It is held in place relative to the stock rails by a gauge plate, a metal plate bolted on the top of the tie under the end of the points. The points are supplied by the track department and are equipped with at least two rods. The rods and the gauge plate usually need insulation (shown solid black above) to electrically separate the two stock rai ls and the machine. A switch machine with its cover removed is shown below.

An electric motor (M) drives, through a gear train and clutch, a large horizonta l bevel gear (B). The clutch is incorporated to protect the motor if the points are obstructed. (Coal from steam engines ja1mning the

41

4 - SWITCHES

points used to be a very common occurrence.) Further protection is provided by the control circuits that usually incorporate an overload cutout. The bevel gear has a pin (P) fitted that engages in the "footprint" shape machined out of the throw rod (Dr). In the plan view below, the cover over the bevel gear has been removed.

LB

p

Dr

As the bevel gear rotates about three-quarters of one revo lution (counterclockwise from the position shown), the rotary motion is converted to linear motion of the throw rod. This throw rod is connected to one of the rods to position the points.

~

i

~

'J

~~

UL ~

~

~ ~

11.

I

I

D LI D D 42

Switch Machines

If the motor fails, the machine may be operated manually by inserting a crank in the hand crank socket at the end of the machine, adjacent to the motor. The action of inserting the crank handle electrica lly isolates the motor so no injury can occur to the person using the handle if the motor suddenly rotated. When the handle is removed, a plunger has to be operated to reinstate the motor connection. It is essential for the interlocking to know the position of the points because signals must not show a PROCEED aspect unless the switch is proved to be set the correct way. A mechanical connection, the point detector rod (De), is taken from an extension piece on the point into the machine to operate point detection contacts. Two parallel banks of contacts may be seen at the left-hand end of the diagrams above. A rocker arm (R) between the two banks (shown in the mid-position) opens and closes the contacts. The four contacts in each bank nearest the end of the machine are used to control the detection circuits back to the interlocking. The other four in each bank are heavy duty and are used for internal switching of the motor drive voltages within the machine. The innermost contacts remain closed while the points are moving, and are broken when the correct position is reached. The rocker arm is operated by the detector rod so that the normal or reverse contacts are only operated when the point is physically in the proper position. Misplacement of the point by a quarter of an inch (about 6 mm) is enough to prevent the contacts operating (and thus preventing the signals from showing a PROCEED aspect).

...,

ql

~

~ ~j

~~

~~

~ ~

....

1

~

I

I I

I

IDI I I I I 43

4 - SWITCHES

Since the early days of railways, there has been a fear of the points actually moving while a train is passing over them in the facing direction. This would result in a derailment and loss of life if it occurred at high speed. Trains trailing through points set in the wrong direction would not normally be derai led, although the switch and machine would be damaged. Switches operated mechanically from a mechanical interlocking by pipe had one lever to move them and a second lever to work a mechanica l facing-point lock. Some early mechanical machines had both functions combined into one device, cal led a switch-andlock movement. Switch machines are a development of the switch-and-lock movement. A mechanical lock is built into the machine and is connected physically to the points by a lock rod (L), which is connected to a front rod. This extra front rod is (usually) provided as part of the signal system and is attached to the points. The locking is achieved within the machine by a bar (LB) parallel with the axis of the machine. This is connected by a short pivoted bar to the pin fitted to the bevel gear. At the other end, under the detector contacts, a part of this bar passes as a close fit through one of two slots cut in the lock rod (one slot when locking the points normal, the other slot when locking the points reverse). The shape of the footprint cutout is such that when the motor starts turning, fi rst the internal lock is withdrawn from one slot, then the points are moved across, and f inally the internal lock is remade in the other slot. The detection contacts operate only when the detection is made up and the locking is effective. In total, there are three mechanical connections between the machine and the points-the throw (or operating) rod, the lock rod, and the detector rod. The detector rod is the rod farthest to the left and is usually connected to the toe of the nom1ally closed switch point (see fo llowing diagram). The lock. rod is next and is connected to the front rod, a rod used to maintain gauge between the two switch points. The front rod is insulated so that the two points are electrically isolated and track circuits can be used in the turnout. The throw rod is the most substantial of the three switch machine rods and is used to move the points from one lie to another. The throw rod is attached to the No . 1 rod, which, as with the front rod, is used to maintain gauge, and is insulated to provide electrical isolation between the switch points. There may be additional rods connecting the two points together; if there are, they are numbered fo llowing the No. I rod.

-.,

I

.,

I "1

~

11.J

f

I

H I

' I

'

DD

D D 44

Switch Machines

Note that the detection contacts are in fact monitoring the position of the points relative to the switch machine rather than the stock rails that they are supposed to be touching. Several terms describe the relationship of a switch machine to the track; sometimes the terms can be confusing. The first term is switch. To be proper, a switch is the assembly of points and stock rails, along with the ties and gauge plates to support it. Frequently, those outside of the track department use the term to apply to the whole turnout, which includes the frog (where the rails cross) and the connecting rails. A main-line power switch machine is the unit with a motor, a gearbox, a throwing and locking mechanism, and (usually) a means of electrically detecting the position of the points (the point detector). To connect the sw itch machine to the points, three rods are usually employed in North America: the operating, lock, and point detector rods. These rods are connected to the points through ( 1) the "front" rod (which is usually for the lock rod), (2) the No. I or "head" rod (for the operating rod), and (3) a point detector connection. The front and No. 1 rods connect the two points together. The entire assembly of the switch machine, rods, and miscellaneous hardware is called a layout. To start with, the switch machine can be mounted on either side of the track. The choice can be based on the space available, the location of adjacent tracks, or the normally closed point. Each railroad has a method for selecting the position of the switch machine. When looking from the point to the frog end of the turnout (in the facing direction), the switch machine is called right hand when it is mounted on the right side of the track, and left hand when it is mounted to the left. The mechanical arrangement of the switch machine is different for left and right layouts. For some switch machines, these differences are small and conversion from left to right (or vice versa) can eas ily be accomplished in the field. For other switch machines, notably dual contro l machines (wh ich are equipped with a hand-throw lever and a "power-hand" selector lever), this conversion can be a complicated process best performed in a shop. Nearly all switch machines sold in North America can be converted from left to right or back, sometimes w ith no additional parts. When a switch machine is connected to something other than a sw itch, "left" or "right" is determined as if it were connected to a switch. For nearly all main-line machines in North America, this is determined by looking at the layout from the point detector end of the switch machine. It does not matter how the turnout itself is confi gured in selecting a right- or left-hand machine. For exampl e, if the turnout diverges (curves) to the right, either a left- or right-hand machine can be used.

The next characteristic required to describe a switch machine is the position of the switch that is considered "normal." The normal pos iti on is defined on the signal plans, and is not necessarily the straight position. For a switch machine, there is genera lly no mechanical difference that is the normal position . There may be a difference in the layout, based on the rai lroad's standards. For example, some compan ies connect the point detector to the near point, and some connect it to the normally closed point. For the switch machine itself, however, this has no effect. The normal position of the switch does have an effect on the wiring of the switch machine. To describe which point is normally closed, several systems are used:

1. T he closed point is designated directly, by saying "right point normally closed" or " left point normally closed." These can be shortened to simply " right point closed" or "left point closed." 2. The closed point can be referenced to the switch machine side, and the terms become "closed point side" or "open point side." These are often shortened to just "closed point" or "open point." These

45

4 ·SWITCHES

mean, for example, that the normally closed point is on the same side as the switch machine, or that the normally open point is on the same side as the switch machine. 3. Another method is to indicate if the normally closed point is near the machine, or if it is the far point. These are termed " near point closed" or "far point closed," respectively.

In the summary table below, the machines in the columns are generally electrically the same; that is, the switch machines are usually wired the same. The layouts in the rows are generally mechanically the same. For example, a LHCP machine (left-hand closed point) is w ired the same as a RHOP machine (right-hand open point); it is mechan ically the same as a LHOP machine (left-hand open point). This discussion is not meant to cover unusual applications (which, in fact, are not that unusual! ). In particular, movable-point frogs (MPFs) and sliding block derails require special considerations, and may be mechanically or electrically different from the basic arrangements described here. This is intended as an introductory summary; reference should be made to the manufacturer's installation and maintenance manuals for the specific switch machine being used. These terms can be summarized in a table, and illustrated with sketches. The arrangement of the sketches matches the main boxes in the table.

Left point normally closed Left side of track

Right point normally closed

Left-hand machine, nom1ally closed Left-hand machine, normally open point side (LHCP); or point side (LHOP); or left-hand machine, left point closed; or left-hand machine, near point closed

Right side of track

left-hand machine, iight point closed; or left-hand machine, far point closed

Right-hand machine, nomrnlly open Right-hand machine, normally closed point side (RHOP); or point side (RHCP); or right-hand machine, left point closed; or right-hand machine, far point closed

0

right-hand machine, right point closed; or right-hand machine, near point closed

0

0

0

46

Manual Switches

Photo 4. A power switch machi ne, in this case, including a switch position indicator

Manual Switches Switch machines are expensive devices, and the control s for a switch machine are a lso not cheap. Many switches are not used enough to justify these costs, and so are hand operated. The symbol for a handoperated switch is simil ar to those for power-operated switches, with the shading removed from the triangles. Most switches in yards are hand throw, but there is rarely any signaling in the yard, so the signal designer is seldom concerned w ith these. Many switches are, however, in signaled territory where tra ins may operate at speed, and the s ignal system needs lo " know" that these switches are in the correct position. The simp lest device is the switch circuit controller. Thi s is similar to the point detector used in a switch machine, but without the power operation and without the locking function. A point detector rod is connected to one point, and the rod operates a cam, which, in turn, operates contacts to detect the position of the points. Most often, only the normal position is detected. If the switch is not in the normal position, the signals will be set to STO P. Switch circuit controll ers may be used to detect the position of

47

4 - SWITCHES

other devices, as well. A derail is an examp le, but many mechanical devices may use a circuit controller to indicate their position. The symbol for a circuit controller is a small "x" next to the switch (or other) symbol.

Photo 5. A hand-operated swi tch with a swi tch ci rcuit controlle r (the small rounded box mounted on the tie between the switch stand and the track)

Between a simple circu it controller on a hand-thrown swi tch and a power switch machine (in level of contro l and cost) is an electric lock. This device may be used to e lectrically lock a hand-throw switch so that it ca nnot be thrown when a train is approach ing. These are available in several models, but the principle is similar in al l. The switch (or other device) wi ll not be able to be operated until a coi l in the lock is energized to release the lock. The coil, in turn, is control led throug h the signal system to be sure the signa ls are at STOP. A time delay can be included so that the lock is not released until a train has had time to stop clear of the switch. If desired, a spec ial hand-throw switch mach ine can be used that includes a faci ng-po int lock similar to a power switch machine, as well as point detection. The symbol for a handthrown switch with an electric lock, and including the symbol for a circuit controller, is:

48

Manual Switches

Photo 6. A hand-throw switch wi th electric lock

49

Chapter S

Wayside Signals

A. J. R. Rowbotham K. D. G. Bisset

51

Introduction

Introduction Signa ls, positioned along the track, pass information to the train operators of passing trains. The signals might be mechanical, where the different physical positions of an arm give different indications, or light signals where different lights give different indications. Mechanical signa ls, known as semaphores, will not be discussed in this document, except as related to terms still in use. A signal has aspects, which are the various ways it can appear to the approaching train. For example, it might be able to display a red aspect, a ye llow aspect, or a green aspect. Each aspect has one (and, on a given section of railroad, on ly one) indication that is the meaning of that aspect. The indication tells the train operator what to do when that aspect is seen. (Beware- the term aspect sometimes is used to refer to the number of lamp assemblies ofa signal. This usage, though technically incorrect and confus ing, does occur even among experienced signal people.) Aspects may be given in different ways. Either different colors (or combinations of colors) or a different pattern of lights (with the actual colors not necessarily relevant) may be used. The former is known generally as a color light (sometimes written colorl ight) signal and the latter is known as a position light signal. Some railroads use the arrangement of lights from a position light signal in combination with the co lored lights, yielding a color-position light signal. Fi nally, the colors can be changed behind a single lens assembly in a searchlight signal, which is really a type of color light signal. T hroughout the world three colors are universally used, although they may be used in different ways in different countries: •Red •Green •Yellow

Stop Proceed Caution (Yellow is never called "amber" in railway signaling.)

Other colors are found, particularly lunar white for RESTR ICTING (or low speed) and sometimes, blue. Blue light has very poor propagation properties through the atmosphere, so it can only be used where it does not have to be seen from a distance. The STOP aspect of a signal is said to be the most restrictive aspect, and the PROCEED aspect is said to be the least restrictive aspect. A CAUTION aspect is between the two, and is consequently less restrictive than a STOP aspect and more restrictive than a PROCEED aspect. When a signal changes from a STOP aspect to a less restrictive aspect, it is said to CLEAR . An aspect less restrictive than STOP, that is, any aspect that is an improvement from a stop indication, is called a PERMISSIVE aspect. Many aspects requ ire two or three lights of different colors to be shown at the same time. In the days of semaphore signals, this meant that two or three semaphore arms were required. These would be mounted on the same mast, at different heights, as illustrated on the front of this chapter. The term "arm" is still used for an assembly of colored lights, only one of which w ill be lit at a time. The term "head" may also be used for this assembly. Where more than one arm or head is required, they are mounted vertically above one another. Vertical, rather than horizontal, mounting of the heads is used to reduce confusion in

53

5 - WAYSIDE SIGNALS

multiple-track situations where signals for the various tracks might be arranged side by side on a bridge structure and where it is important that train operators accept and act only on the indication meant for their track and not for one adjacent to the track they are on. Note that some railroads will use a sli ght offset of the heads (or of marker lights) to further define the aspect, but that the vertical mounting of the signal heads is still used. Depending on the signal rules of the railroad, it may not be necessary that a light be displayed on every arm. These instances are defined in the railroad 's Aspects and Indications. Signals may also use markers (single lights arranged above or below, and sometimes to either side of the signal head that further define the aspect); markers may be either on or off.

Types of Signals Color Light The color light signal head consists of separate lightproof compartments each holding a lamp, lamp socket, and lens assembly. Each lens assembly usually has two lenses, an inner lens and an outer lens, referred to as a doublet arrangement. The lenses are molded as a pri sm, with the prismatic featmes of the lenses serving to fo cus the light in a narrow beam to provide a li ght that can be seen for a distance. The inner lenses are suitably colored to produce red, yellow, green, or lunar white light, taking into account the yell owish li ght emitted by the lamps. The lamp filament must be accurately placed with respect to the lens to ensure efficient performance. Signal lamps are usually manufactured with smaller tolerances on the location of the filament with respect to the base than normal domestic or industrial lamps. Mirrors must not be used to enhance the light output. This is to prevent the danger of light from the sun or a locomotive headlight entering the signal head through, say, the green lens, reflecting back from the mirror and then appearing to the train operator to be a green aspect. The inside of the signal head is painted flat black to sim ilarly reduce any chance of reflections. Hoods are provided to reduce the possibility of the sun shining directly on the lens, and each signal has a background to make it easier to spot from a distance. Even w ith the precautions described above, it is sometimes possible, due to the location of a signal, for sufficient light to be reflected from the sun or from a locomotive headlight for an unintended aspect to be visible to the train operator. Thi s is called a phantom and presents an extremely dangerous situation. Baffles, called "phan-kills," can be installed in the lamp housing to reduce the possibility of a phantom. Some railroads only install phan-kill s in lamp housings where there has been a reported problem, or where pre-siting reports indi cate that a possible problem w ill exi st. Other railroads order signal heads from suppli ers with phan-kills installed in all lamp housings to preclude any such situation. The most restrictive aspect of a physical signal is usually placed nearest the train operators' eye level, and thi s usually means the red is the lowest aspect. The yellow is usually placed between the red and the green on signals with lenses in a straight line. When lenses are arranged in a triangle, the red is again usually the lowest lens. The yellow and green are placed above the red, but there are differi ng standards for which lens is in which position.

54

Color Light

The actual colors used are specified by AREMA (formerly AAR, formerly ARA-American Railroad Association-and the Railway Signal Association); the tolerances are small, so there is no possibility of confusion among the colors. When color light signals were first introduced around 1905, they were a replacement for the semaphore signal. Semaphores had colored roundels (plain glass, not a lens) to provide the night aspect, and these were illuminated with oil (kerosene) lamps. The oil lamps did not need to be very bright, because they were only needed at night- the position of the semaphore arm was sufficient for the daylight aspect. In a color light signal, though, the light had to be seen in all conditions. Commercial power was not available in many areas, and where it was, the reliability was poor. Even today, commercial power generally is not reliable enough to be the only source of power for most situations. Batteries are used to supp ly the current for the signal system; in the early days, e ither primary cel ls (nonrechargeable) were used, or rechargeable cells were taken to a central location fo r recharging. Since the lamp in a signal used more power than the rest of the system, great attention was paid to the efficiency of the lamp assembly. Typ ica l signal lamps today are in the range of 13 to 25 watts each and, with the efficient lens assembly, can be visible for nearly a mile. A few installations are beginning to use light-emitting diodes (LED 's) to produce the colored light. LED's have a much longer life than incandescent lamps, and thus reduce maintenance costs and the costs associated w ith failed (dark) signals. LED's present their own design challenges to the signal engineer. Because each LED light unit is constructed from arrays of smaller LED 's (arranged in series and parallel circuits) precisely determining what constitutes a light-out condition is difficult. Light-out relays for incandescent lamps merely detected a complete circuit path through the filament of the bulb and acted to downgrade approach signals in the event that that circuit path was interrupted. A typical doublet lens assembly looks like this, with the general path of the light rays shown. (This di agram does not indicate the exact amount of refraction at particular points.)

55

S - WAYSIDE SIGNALS

The next two photos show examp les of two designs of color light signals.

Photo 7. A color light signal with vertical lenses and two heads

•

•• • ~

"'

-

Photo 8. Color light signals with lenses in triangular arrangement; each of the two signals has three heads. Notice that some heads can only di splay one or two colors.

56

Searchlight

Searchlight Strictly speaking, a searchlight signal is a form of color light; that is, it shows aspects only by the colors of the lights. A searchlight signal uses only one lens for each head, and changes the color of the light passing through that lens. The traditional searchlight uses a mechanical device (the mechanism) to move fi lters into the light path. The lamp is placed at one focus o f an e ll ipsoidal mirror (one of the very few uses of a mirror in a signal), and one of the fil ters is placed near the other focus of the m irror. A lens gathers the light; the light then passes through a further lens (or lenses) to produce the required narrow beam. Any sunlight (or light from a head light) entering the signal will pass through the fil ter before being reflected at the mirror, so a falsely colored aspect is not possible. The filters may be moved by a device similar to a polar relay; energy of one polarity swings the filter holder (the spectacle) in one direction to display, for example, a yellow aspect; and energy of the opposite polarity w ill swing the spectacle in the opposite direction for a green aspect. Removing energy allows the spectacle to fa ll to the middle, where a red fi lter is placed in the light beam. Contacts are provided in the mechanism to detect the pos ition of the spectacle. The entire mechanism is in a sealed housing, and is treated as a vital relay. Some newer searchlights use a separate lamp and filter for each color, and foc us the colored light on a fiber-optic assembly. The fiber-optic assembly gathers the light from all lamps to a single spot, and the li ght is then focused by clear lenses. Lens

..,..

Lamp

Color Disc (Roundel)

Spectacle

Pivot

Diagram of searchlight basic design

57

5 - WAYSIDE SIGNALS

Photo 9. A three-head searchlight signa l

Positon Light Among the first objections to color light signals was the reluctance to eliminate the semaphore blade position display. To answer this concern, the Pennsylvania Railroad developed the position light signal. Aspects are displayed by a row of three lights- horizontal for stop, vertical for PROCEED, and diagonal (lower left to upper right) for CAUTION. These corresponded with the positions of an upper-quadrant semaphore with the blade pointing to the right, which had been a common arrangement in the United States. A fourth aspect, lower right to upper left, correspond ing to a lower-quadrant semaphore, was used for a RESTRICTED or RESTRJCTED PROCEED aspect for those situations where trains were allowed to enter an unsignaled track, or an occupied section, such as a storage track or platform. The lights were originally

58

Color-Position Light

all colored a pale yellow. chosen for maximum fog penetration. The center light, wh ich is always lit, is a common "pivot" light, with the remaining lights arranged in a circular fashion around the pivot light. One advantage of the position light signal is its inherent redundancy; if one lamp should fa il, the aspect is still clearly apparent from the remaining two lamps.

A single-head position light signal

Color-Position Light A natural extension of the position light signal resulted from using appropriately colored lenses in place of the pale yellow, and eliminating the "pivot" lamp. Thus, the STOP aspect consists of two red lights on the horizonta l, the PROCEED is two green lights on the vertical, and the CAUTION is two yellow (real yellow!) lights on the diagonal. The color-position light also has redundancy built in to the main aspect, since two lamps are used for each display. In an attempt to simplify installation of color-position light s ignals, multiple heads are avoided. Instead, qualifying markers are used to indicate whether the aspect di splayed is for a high-, medium-, or low-speed route, or for other indications. Markers are single light units (either yellow or lunar white), similar to the light units in the main head, with small backgrounds. They are placed above and below the signal head, and may be offset to the right or left to carry different meanings. The aspects are usually arranged so that failure of a marker will not cause a Jess restrictive aspect. A strength of both the position light and color-position light signals is the fact that " light-out" circuitry was not required for their application. Signals that depend upon a s ingle lamp to provide an aspect normally require some type of light-out protection to assure that APPROACH signa ls are downgraded in the event that an aspect is not avai lable because of a failed lamp. Position light and color-position light signals, however, depend upon multiple lamps to provide an aspect. Train operators (drivers) are instructed to report any aspect that is not perfectly displayed, and corrective action can be taken before a signal goes completely dark.

59

5 · WAYSIDE SIGNALS

Photo 10. A color-position light signal with two markers

Dwarf Signals In congested areas where train speeds are low, miniature versions of the above signals may be used; these are called dwarf signals. The lenses are usually sma ller and less efficient, but with low speeds, long sighting di stances are not required. The miniature position light signa l uses only two lights in a row, instead of the three in the full-size version. Dwarf signals often cannot display full high-speed PROCEED aspects. Dwarf signals are usually mounted close to the ground, with either a short or no mast. Thus, a dwarf signal can be installed where there is no room for a full-sized signal and mast.

60

Symbols

Signal Arrangement and Mounting An individual signal consists of one or more signal heads or arms, and a supporting structure. The structure most common is a simple mast, with the signal heads mounted to the mast, either directly in front or to one side. The heads are provided with adjustments in the mountings so that the lig ht beams can be accurately aligned along the track for best sighting; that is, aligned so that the operator of an approaching train will see the signal as soon as possible, and will not lose sight of the signal while approaching the signal. If the track is curved, spreadlight or deflecting lenses might be required to make the beam of light wider to cover more of the track in approach to the signal. Other possible mountings include the signal bridge, which spans the track (or tracks), and wh ich might carry several individual signals; and just a foundation with direct mounting of the signa l, for ground or dwarf signals.

Symbols To show signals on track plans, the symbols defined in the A REMA Signal Manual, Part 16.2.1, are used. For the purposes of this discussion, a few of these symbols will be sufficient, and will be explained here.

1

Start with a signal mast:

Add a symbol for each color (or equivalent) that the signal head (or arm) can display:

l r

GREEN

YELLOW

RED

LUNAR WHITE

The normal (not necessarily usual! ) aspect is denoted by a heavy line, and a signal can have more than one head or arm. The identifying number is placed next to the signal. These symbols are combined to give, for example:

Here, Signal 14 has a single arm, or head, implying that it can display only one light at a time. Signal 14 can display a red, yellow, or green aspect, and the normal aspect is green. Signal 12 has two arms, so it can display two lights at the same time. The top arm can display red, yellow, or green, while the bottom arm can display red or yellow. The normal aspect is red over red. The small segments of a disk at the base of the arms indicates that this signal is semiautomatic; that is, it is controlled manually, such as by a lever, as well as automatically by the track circuits beyond the signal. This signal can be cleared by the towerman (or other operator), and will be put back to STOP when the train occupies the track circuit beyond the signal. The solid segment in the symbol also shows that the s ignal control includes stick control. This means that the signal will not automatically re-clear as the train continues along the track; the towerman

61

5 - WAYSIDE SIGNALS

must again request the signal to clear for each train. This is the typical symbol used for a controlled signal at an interlocking. Signal 14 here is an automatic signal , controlled by track circuit occupancy but with no controlling lever. On track plans, the tracks usually run horizontally across the page rather than vertically, so the symbols just drawn have to be rotated through 90 degrees, resulting in the mast of the signal being shown parall el to the track. Signals are usually physically placed on the right-hand side of the track to which they apply. A signal may be placed on the left-hand side of the track, as long as there is no possibility of confusing the train operator. A conventionally placed three-aspect signal wou ld be shown:

T his is a signal for traffic moving from left to right (i.e., the bottom of the mast is met first in the direction of travel). Note also that the signal has to be viewed from the track to determine the colors represented by the symbols. A signal for traffic moving from right to left, conventionally placed on the right-hand side of the track is shown:

If the above signals were placed on the left-hand side of the track (the exception rather than the rule), then the symbols would be, respectively:

Where qualifying signs are important, such as number plates, the signal symbol can be modified to show such fittings. For the example of a number plate, the symbol would be:

The shape of the symbol would match the method used on the railroad, and so this modification will be different on different railroads. Other signs or symbols may be used to modify signal aspects, but these vary on different railroads.

62

Speed and Route Signaling

Signal Aspects and Indications To understand how wayside signals are used, one should know w hat signal aspects and indications are available. The railroad's rulebook or special instructions will have a d iagram of the aspects used by that railroad, along with the name and indication of each aspect. It is important that this information be ava ilable and understood when the initial signal layout is designed. Be aware that a particular project might be able to use only a subset of the aspects shown in the rulebook, because different sets of aspects may apply to different areas of the railroad. This can occur particularly where a railroad is a combination of several formerly independent railroads. A lso be aware that if the insta llation involves more than one rail road, the aspects and indications will probably be different on the various railroads. An appendix to this chapter is provided with some example signa l aspects and indications, along with some other example-associated operating rules. Remember, these are artificial, produced only for the purposes of this docum ent. They are based on actual rulebooks, but they are considerably simpler than actual rulebooks. The ru le numbering is completely artificial, partia lly to emphasize that there are many other rules, and partially to avoid possible confusion with actual rulebooks. The number of signal aspects shown is much smaller than the typical rulebook. The appendix is provided so that the reader can see what a sample set of signal rules might look like; if an actual rulebook or special instructions is available, it is strongly suggested that the corresponding rules be located and reviewed. In broad terms, wayside signaling falls into two major categories: block signaling and interlocking signaling. Block signals govern the use of blocks, or sections of (generally) plain track, and are used to space trains apart. Interlocking signals govern the use of routes through turnouts and crossings, where the switches and signals are controlled together and interconnected (interlocked) to assure that conflicting movements cannot occur. B lock signals can thus use fairly simple aspects: STOP, CAUTION (or APPROACH) and PROCEED. Interlocking sig nals, on the other hand, must also disp lay information on how fast the train may go, or on what route the train will be traveling. Interlocking sig nals can therefore display many more aspects and indications (in general) than block signals.

Speed and Route Signaling There are, throughout the world, two basic signa ling strategies: speed signaling, and route signaling. With speed signaling systems, the signals give the train operators additional information about the range of speeds at which they sho uld operate. In some systems, speeds are defined merely as " low," " medium ," or " high," and the indications are defined in these terms. This leads to a system needing perhaps 14 or 15 different aspects to be differentiated by the train operators. In some more modern systems, which tend to be simpler, the ach1al speed is indicated to the train operators numerically. The signal will indicate to the tra in operators the speed allowed on the established route, but wi ll not necessarily indi cate which route is established. This is particularly true where several routes have the same o r simi lar max imum speeds . With route sig naling, it is up to the train operato rs to drive at the correct speed, using their intimate knowledge of the line and its speed limits. (They must become very fam iliar with a ro ute before being allowed to operate over it.) At junctions, it is on ly necessary to indi cate to them which route is set. As a resul t, route signaling generally leads to a simpler system of aspects.

63

5 · WAYSIDE SIGNALS

U.S. freight railroads located east of the Mississippi generally use speed signaling; those located west of the Mississippi and transit systems often use route signaling. Both the Canadian Paci fie and the Canadian National use the aspects and indications of the Canadi an Rail Operating Rules that specify a scheme of speed signaling. The scheme used differs between railroads, and sometimes between different divisions of the same railroad. (This latter point is mainly due to mergers and purchases among railroads.) There are a few "standard" sets of rules (including the signal aspects and indications) used by severa l rai lroads, but these became complex enough that the signal rules are moving out of the rulebook and into the timetable or special instructions. STOP

Versus STOP

AND PROCEED

B lock signals (sometimes also called intermediate signals) are automatic signals. No person controls the signal, and no one monitors the condition of the signal. A fai lure will only be discovered when the train operator sees the signal. If a track circuit fa ils, for example, the only indication is the red signal. The train will have to stop, but consider what happens then. The soluti on is to allow a train to pass a red block signal at restricted speed (a speed that will allow the train to be brought to a stop within the train operator's range of vision of the track ahead, usually not more than 15 or 20 mph). This speed has to be 111aintained until the train passes a more perm issive signal. The aspect is known as STOP AND PROCEED, although different railroads use different names and have different requirements. Of course, there must be so111e means for the train operator to di stinguish which signals may be passed when displaying red; the most common way is by putting a number plate on the block signals (permissive signa ls) but not on s ignals where the tra in must remain stopped (absolute signals). Where several routes are available, the signal must be able to display STOP (without the permiss ive feature) to prevent unauthorized train movement. Part of the consideration of STOP versus STOP AND PROCEED is the ability of a person to monitor the operation. For automatic signals, indications are not generally provided to allow someone to watch the condition of the track circuits and signa ls. At an interlocking, on the other hand, an operator (towerman or dispatcher) is usually controlling the signals, and so indications of track circuit and s ignal condition are provided. The dispatcher can therefore instruct the train operator if there has been a fai lure, and give verbal permission to move the train. This verba l permission must, of course, be g iven carefully to avoid any misinterpretation. Signals can be controlled in two ways: automatically, using track circui ts only; and manually. The latter often includes track circuit controls as well (known as slotting), but someone determ ines if the signal is to be cleared. Automatic signals are also known as block or intermediate signals (the last term because the signals are between, or intermediate to, controlled signals). Controlled signals are used at interlockings, and are often called home signals. The types of control, and the most restrictive aspect typically used, are summarized in the following table . Most Restrictive Aspect

Absolute [and

(STOP

Type of Signal

STAY])

Permi ssive (STOP AND PROCEED

Controlled (Interlocking or home)

Automatic (Intermediate or block)

All No number plate

A few; APB, no number plate

None

Most; usually have number plate

or RESTRI CT ING)

64

Signal Aspects

There are interlockings that operate automatically, based on occupancy of track circuits. The associated signals are still interlocking signals, and the controls are essentially the same as a manually controlled interlocking. The automatic interlocking controls replace or supplement the manual controls, and the interlocking principles remain the same.

Signal Aspects Many different aspects are possible for interlocking home signals-so many that it is impossible to describe them all here. Different railroads have different aspects and indications; and, due to mergers and purchases, different aspects and indications may well be on different areas of the same railroad. A few generalizations may be made, with the understanding that these are only broad genera lities, and the rules for the specific application must be reviewed. Home signals often have more than one arm. The top arm generally governs high-speed movements, the middle arm generally governs medium-speed movements, and the bottom arm (where used) governs low-speed movements. In some instances, the middle arm might be omitted if there is no medium-speed route. For each arm, the basic colors are used: red for STOP, yellow for CAUTION, green for PROCEED, and lunar white for RESTRICTING. Sometimes, flashing is used for a less restrictive version of the basic aspect; for example, some railroads use a flashing red for RESTRICT ING in some instances instead of lunar white. For a typical simple aspect, the appropriate arm will display a CLEAR aspect (yellow or green) while the other arms will display red. Thus, a red over yellow over red might be used for a DIVERGTNG APPROACH aspect; the indication would be something li ke PROCEED on diverging route at prescribed speed through turnout, not exceeding 35 mph, prepared to STOP at next signal. The preceding discussion applies to color light and, to a certain extent, position light signals. For colorposition light signals, the principle is different. Only one arm is used to display the basic red, yellow, green, or lunar white. Markers (auxi liary lights) are illuminated above or below the main signal head to modify the basic aspect to produce indications similar to the multiple arms of a co lor light signal. In any case, review the signal aspects and indications for the railroads involved. There are many exceptions to these generalizations, and there are usually additional aspects not mentioned here. Be sure that the sequence of aspects is understood, as well as the implications of dark or fa iled markers or arms.

65

5 - WAYSIDE SIGNALS

Appendix 53. * Restricted Speed Where restricted speed is required, movement must be made at a speed that will allow stopping within one-ha If the range of vis ion, short of a train or railroad car, men or equipment fouling the track, switch not properly lined, or a broken rai I. Do not exceed l 5 mph.

112. * Improperly Displayed Signals Except where shown in Rules I 0 I through 107 and in the specia l instructions, if a lig ht is missing, or dark, or a white light is displayed where a colored light should be, a block or interlocking signal must be regarded as displaying the most restrictive aspect that can be di splayed at that signal. Improperly displayed signals must be reported promptly to the dispatcher. Rule

10 1

102

Name CLEAR

Aspect

Indication Proceed

r2

AD\"ANCE APPROACH

0

G R

0

y y

2 103

Proceed prepared to slop at the next signal.

APPROACll

l 104

y

y

0

2

D!YERGING l\PPROl\Cll

0

2 105

Proceed prepared lo stop at second signal. Trams exceeding 40 mph must immediately begin reduction to 40 mph

R Proceed on di\·erging route. not exceeding prescribed speed through turnout, prepared to stop at next signal

R y

RESTRICTING

Proceed at restricted speed, not exceeding prescribed speed through turnout.

0

R

2 LW 106

STOP AND PROCEED

Stop. Then proceed at restricted speed lo U1c next signal.

'.r

(number plate)

107

STOP

l

R

Stop

0

2

R R

(no number plate)

*Rule numbers are arbitrary; rules are based on actual rules but have been modified.

Example rules, for discussion only. Not to be used for design!

66

Chapter 6

Relays and Relay Logic

K. D. G. Bisset A. J. R. Rowbotham

67

The Relay

Introduction For many decades, the interlocking function has been carried out by relay circuitry. An interlocking might consist of many hundreds or even thousands of relays appropriately interconnected to put together the safety logic conditions required. Although microprocessor-based vital systems have been in use since the mid- I 980s, they have not become universally accepted in new applications. Relay circuits are sti ll installed on many railroads and transit systems. Thus, relay circuits are still important in the ir own right. Even with microprocessors, the logic uses the same fundamentals as relay circuits. Vita l microprocessor logic is largely a direct translation of the relay logic. It is therefore important to understand the functions applied in relay logic, even if the application wi ll use a microprocessor.

The Relay A relay cons ists of a number of switches, or contacts, and one or more coils. The contacts are all operated simultaneously by applying a vo ltage to the coil. Each contact consists of two flexible contact arms, one of which may be fitted with a pad of graphite impregnated with silver.

1 2 The pair of arms act as a switch; if the two are not touching, as in the diagram above, the contact is open and no current can flow through the contact. If the two are pushed together so they touch (at the righthand end in the diagram), the contact wi ll be closed and current can flow. The movement of the arms is provided by the action of an electromagnet, formed by a metal core within the coil of the relay. The diagram below shows the principle of this action. It is assumed in this diagram that no current is flowing through the coil E/F. At the top are two contacts- A/B is shown open, while CID is shown closed.

A

\:::(

B

l

c

Pivot

D

E Armature

F

Coil

69

6 - RELAYS AND RELAY LOGIC

If a voltage is applied to the coil then the resultant magnetic force attracts the pivoted armature, which, in turn, pushes up two of the contact arms. This results in contact AIB being closed and contact CID being opened. The state of the contacts can thus be controlled by the state of the coil.

Terminology A relay is said to be energized if the current is flowing through the coil, and de-energ ized if it is not. These are the formal terms, but other terms are commonly used. A relay is said to have "picked up" when current is appl ied to the coil, and to have "dropped away" when the current is removed. The terms picked (for energized) and dropped (for de-energized) are often used, as are the even simpler plain up (for energized) and down (for de-energi zed). The diagram with A IB and CID contacts shows that there are two types of contacts involved. In other professions, it is common to use the terms "normally open" or "form A" and "normally closed" or "Form B" to describe contacts NB and CID respective ly (assuming that the relay is normally de-energized and picks to fulfill its function). To ensure the required safety, however, it is common in signa ling for some relays to be normally energized, and to drop to fulfill their function. The term "normally" thus becomes meaningless and different terms have to be used. Like a lot of things in signaling, the terms are now anachronistic, but when they were introduced, they were meaningful. The type of contact closed when the relay is de-energized (CID in the diagram) is known as a back contact because in early relay design they were at the back of the relay. Conversely, the type of contact that is closed when the relay is energized (NB in the diagram) is known as a Font contact. The contacts as shown are also known as independent back or independent Font contacts. The moving part of the contact is known as the heel. Frequently, a front and a back contact share the same heel. This is known as a Font-back, transfer, or dependent contact, and is illustrated below. In other industries, this may also be known as a "Form C" contact.

......==========~=•=

~'\Ji!lllli!!tl==-=-====----,_,,

C:::--~= ~-==a=======c=---";; -'

Types of Relay Vital signal relays seem to have been developed largely as a result of the track circuit, invented in 1872. By 1900, relays seem to have become fairly commonp lace and, by 1905, had started to look much like the typical shelf-type relay (as illustrated on the front of this chapter). The shelf-type relay consists of an insulating plate with terminals above and glass-enclosed contacts below. This design is sti ll manufactured for replacement purposes, and will be encountered in some existing installations. Today, the plug-in sty le of relay is typical, and is manufactured by Union Switch and Signal (PN-150 and PN-250 Series), Safetran (ST-I and ST-2 Series), and ALSTOM (formerly General Railway Signal, BI and B2 Series). Relays from the last two companies are of very similar design, some types being interchangeable. The PN-150 and PN-250 series are of a different desi gn, and are not interchangeable with the other two manufacturers' series. The BI and B2 Series w iII be described here.

70

Types of Relay

B2 relays are twice as w ide as the BI, and can contain twice as many contact assemblies. Each style is avai lable in many different types, and several varieties of each type. The different types of relay include: • Neutral - The basic relay with no special features. • Biased - Relays that are magnetically designed so that the relay will only operate when the current is passed through the coil in one d irection. Current flowing in the opposite direction will not operate the relay. Also called biased-neutral, as distinguished from polar. • Heavy duty - Re lays that are fitted w ith heavy-duty contacts, usually for sw itch machi ne operation. The contacts have a magnetic blowout feature to stop arcing when they open while carrying a heavy current. Slow pick-up - Relays that are magnetically designed so that the contacts do not start to open or close as soon as the relay is energized. • Slow release (or slow drop away) - Relays that are magnetically designed so that the contacts do not start to open or c lose as soon as the relay is de-energized. • Track relay - Relays that are specially designed with a low-resistance coil to work as part of a track circuit. • Code responsive - Relays that are fast acting to operate with circuits where the current is switched on and off rapidly. • Code transmitter - A relay that opens and closes contacts at certain rates, usually 75, 120, or 180 codes per minute. • Time element - A relay with front contacts that close a preset time after the re lay is energized; can be set up to 20 minutes. The ti ming device is either mechanical or clockwork, although the newest time-element relays are microprocessor-based. • Thermal timer - A type of time-element relay where a voltage is applied to heat a bimetallic strip. As the strip expands, the movable heel of the contact is moved until it touches the front. Time has expired when the contact is closed. These therma l timers are used for timing events lasting from a few seconds to 5 minutes. • Light out - Relays with low-resistance coils for detecting filament fai lures in signals. • Flasher - Relays to flash lights, either for highway crossings or for wayside signals. • Power transfer - Relays that are designed with particular characteristics to transfer power to batteries if the commercial power feed should fa il. • Polar - Relays that close one set of contacts when current of one polarity is applied, and another set when current of the opposite polarity is applied. • AC vane - Relays that are particularly used for alternating current track circuits, but used for AC line circuits, also. The basic B 1 relay can have up to three contact stacks (columns), each with up to six contact springs. B2 relays can have up to six contact stacks. There are many arrangements of contacts; for a B 1, one typical arrangement is 6 FB (for six dependent front-back contacts). Often, more front contacts are needed than backs, and many circuits do not require dependent contacts, so another arrangement is 4 FB-2F-1B (for four dependent front-back contacts, two independent fronts, and one independent back). Typically, two coi ls are prov ided, but in some relays there is only one. Where two coi ls are provided, they are usually connected in series, but some applications may require them to be connected in para llel, or connected independently.

71

6 - RELAYS AND RELAY LOGIC

The connections to the contacts and coil are numbered on a coordinate system. The columns of contacts are designated I, 2, and 3, (and 4, 5, and 6 for a B2), which numbers are used for the first digit. Within each column the connections are numbered from bottom to top, 1 through 6, for the second dig it of the contact number. Thus, the third contact spring up from the bottom of the first column would be designated 13. The number for a front is always one greater than its associated heel, and for a back is one less. The coil connections are designated by letters A, B, C, and D in combination with the column number. Separate wire connections are also provided, and are lettered E along with the column number. All modern relays (Type B, PN- 150, and ST) plug in to suitable plug boards mounted on equipment racks. The wiring is held in the plug board, so replacing a re lay does not disturb the wiring. Each wire is terminated with a crimped connector, or terminal, which, when inserted in the plug board, locks in position. A special tool is needed to release the term inal from the plug board when a wire has to be removed. It is essential that the type of relay specified in the drawings is plugged into the relevant plug board. Every relay is fitted with a registration plate with several holes, arranged in a unique pattern for each re lay type. Each plug board has to have the matching registration plate installed, with the corresponding pins.

Relay Circuit Diagrams In signaling practice, the same circuit diagram is used for a number of activities. There is no requirement to draw different forms of the same circuit for different activiti es. These activities include : • • • • • • •

Design C hecking Installation Testing Commissioning Maintenance Decommissioning

The same diagram may include all the information for these activities, so diagrams may actually be true wiring diagrams rather than schematics. Detailed draw ing standards depend on the customer's requirements, so what is shown here is only one possible method of showing circuits.

Symbols There are two bas ic systems of symbols used for relay c ircuits. Written circu its use straight lines, with contact and coi l symbols added along, above, or below the line. So-called drop contact symbols are described in the American Railway Engineering and Maintenance-of-Way Association (AREMA, formerly AAR) Communications and Signal Manual of Recommended Practices (usually called the AREMA Signal Manual). This discussion w ill use the latter system. The basic symbol for a relay coil is remini scent of the appearance of a co il wound on a spool. With the two coils typical of a B relay (either Bl or B2), the symbol becomes a bit more complicated to show the connections of the two coils and the test terminals. Referring to the figure below, the symbol to the right shows the two independent coils of the relay wired in series.

72

Symbols

1,~

Typical Relay Coil Symbol

Type B Relay Coil Symbol

Again unlike other industries, signaling practice shows the relay contacts in the normal position. Thus, there are symbols for front, back, and transfer contacts, in both normally energized and de-energized positions, making six different symbols. lf one considers that the contact cou ld face in either direction on the circuit drawing, a total of twelve symbols for basic contacts would be needed. Only the six basic symbol s are shown below, as the other six are simply mirror images of these. These symbols show the heel to the right of the contact.

Front

• t

• •

Back

Front-Back

Normally Energized

Normally De-energized

The contacts discussed here are the most common ly used type. There are also heavy-duty, make-beforebreak, polar, timer check, and other contacts. Symbols for other types of contact are shown in the AREMA Signal Manual. The name of the function that a relay is performing is inserted next to or above the coil symbol, and above the contact symbol. The contact and coil numbers may also be shown.

NAME

NAME

·-- ~-- -

NAME

32

73

15

6 - RELAYS AND RELAY LOGIC

A series of letter codes are used, that when applied in combination, describe uniquely the function of the relay. All contacts of a given relay will have the name of that relay. A sing le installation must not have two relays with the same name. The final item needed to draw simple circuits is the power supply. For many years, the only practical source of e lectricity was a battery; commercial power was e ither not available at all, or was very unreliable. Thus, the DC power typically used fo r relay circuits is commonly called battery power, irrespective of the actua l device supplying the power. The typical power supply voltage used in North America has been 12 volts DC, and this continues in nearly all installations today. Other voltages may appear, for example, 24 volts may be used to power switch machines, or in a "split battery" configuration, to power nonvital systems. Since there may be more than one " battery," the nomina l voltage may be used as part of the name. The positive side of the supply is thus commonly Bl 2 (for positive 12 volt DC), and the negative side is N l2 (for negative 12 volt DC). Where the supply is 12 volts, the voltage is often omitted. Sometimes, an arrow tai l is used to designate the positive connection, and an arrow head for the negative. The examples here will just use the energy designation.

B

)>----

N )

B

N

Simple Logical Circuits The circuits to be described are not necessarily practical circuits, but are chosen to show the logic utilized.

Example 1 Suppose it is required that a Rel ay R is controlled by Relay A, to the requirement:

Relay R to be energized only when Relay A is energized. This statement could equally have been written as:

Relay R to be de-energized only when Relay A is de-energized. Both statements represent the same logic and result in the same circuit. A logic expression defining a circuit can either define the conditions for energizing the relay, or for de-energizing the relay. The requirement is to control Relay R by contacts of Relay A, so the coil of Relay R can immediately be drawn toward the right of the circuit, and the power feed toward the left. B

N

R

74

Simple Logical Circuits

Between the battery feed and the coil a contact of Relay A is required, so that the logic detailed above is achieved. A decision needs to be made-is it a front contact of Relay A that is required, or is it a back contact? For Relay R to be energized, the current must be flowing, so the chosen contact of Relay A must be closed to com plete the circuit. The requirement is for Relay A to be energized to energize Relay R, so the chosen contact of Relay A must be closed when the relay is energized (i.e., a front contact).

B

A

N

~

R

(Relay A is shown here as normally de-energized, and thus Relay R will also be normally de-energized. Relays A, B, and C will be shown normally de-energized in all of the following circuits.) This completes the circuit- a front contact of Relay A controls Rel ay R. In this situation, Relay R is said to be a repeater of Relay A because it behaves in the same way.

Example 2 Suppose it is required that a Relay R is controlled by Relay A, to the requirement:

Relay R to be energized only when Relay A is de-energized. Th is statement could equally have been written as:

Relay R to be de-energized only when Relay A is energized. Again, both statements result in the same circu it. Using the same arguments, the current used to energize Relay R must flow through a closed contact of Relay A, this time only while Relay A is de-energized- so a back contact.

B

A

N

~

R

(In any case, there are only two types of contact to choose from, so if it is not a front contact, it must be a back contact!)

Example 3 In this exercise, three relays are involved. The logic required is:

Relay R to be energized only when R elay A AND Relay B are both energized. To write this the other way, starting with "Relay R to be de-energized ..." is not quite so easy. The correct logical expression is actually:

Relay R to be de-energized only when Relay A OR Relay B is de-energized.

75

6 - RELAYS AND RELAY LOGIC

Note that the AND in the "energized" requirement changes to OR in the "de-energized" expression. In signaling logic, it is understood that the "OR" condition above does include both relays de-energized as well (i. e., it is an "inclusive OR"). In this example, a contact of Relay A and a contact of Relay B will be in the circuit for Relay R. In order for the current to flow, all contacts of Relay A and Relay B in the circuit must be closed (and only when Relay A and Relay B are both energized) so a front contact of Relay A in series with a front contact of Relay B is required: B

N R

Note that both Relays A and B have to be energized for the current to flow, but either relay de-energized (or both) will break the circuit. Swapping the position of the two contacts around would not alter the behavior of the circuit- the position of contacts in a series circuit is not important in theory.

Example 4 Again with three relays. The logic required is:

Relay R to be energized only when Relay A OR Relay B is energized. Or, writing the same logic as a " de-energize" expression:

Relay R to be de-energized only when Relay A AND Relay B are both de-energized. Note the change from OR to AND to write the "de-energized" expression. For this circuit, the OR means that alternative paths are required to energize Relay R, so the contacts of Relays A and B need to be in parallel, and schemati cally, the circuit would be: N

R

Note that either of Relays A and B (or both) have to be energized for the current to flow, but only both relays de-energized will break the circu it. Swapping the position of the two contacts around would not alter the behavior of the circuit- the position of contacts in a parallel circui t is not important in theory. As drawn above, the circuit is shown schematically; it is not a wiring circuit. How does the installer join the wires together to make the parallel path? The design of the relay w iring terminal allows for two wires to be inserted. Thus, one way of joining w ires is by a relay terminal. In the schematic diagram above, the wire on the front (left-hand side) of the Relay B contact could be connected as a second wire to the power supply feed, or a second wire to the front (left-hand side) of the contact of Relay A. Similarly, the wire on the heel (right-hand side) of the Relay B contact could be connected as a second wire to the left-hand end of the coil of Relay R, or a second wire to the heel (right-hand side) of the contact of Relay A.

76

Further Simple Logical Circuits

Summary of Logic All logic is built up from ANDs and ORs resulting in series or parallel circuits. The relationship can be summarized in a table:

To energize a relay To de-energize a relay

AND condition

OR condition

Contacts in series

Contacts in parallel

Contacts in parallel

Contacts in series

Further Simple Logical Circuits Example 5 Consider two requirements:

Relay X to be energized only when Relay A AND Relay B are both energized. and

Relay Y to be energized only when Relay B AND Relay C are both energized. At first sight, the two circuits would be:

B

A

N

B

x

B

B

c

N

y

However, by altering the order of series contacts m one of the circuits, the two circuits may be economically combined: B

B

A

N

x c

N

~

y

77

6 - RELAYS AND RELAY LOGIC

Example 6 This exercise introduces a new and important concept. Cons ider the requirement:

Relay R to be energized when a button is pushed, and to remain energized when the button is released. The symbol fo r a normal "push to make" button is:

The circuit can be started thus: p

B

N

----------~

R

so Relay R will be energized as long as the button is being pushed, but will de-energize as soon as the button is released. An alternative path needs to be provided to the coil of Relay R, but only once the relay is energized. This path can be provided by a front contact of Relay R itse lf, in parallel with the pushbutton contact: B

N

~~~----------

R

In the quiescent state, Relay R w ill be de-energized, so the front contact will be open. Consequently, the only path to the coi l is through the pushbutton. As soon as Re lay R is energized, the front contact will close to provide the alternative path, keeping the relay energized when the pushbutton contact is no longer closed. Of course, the relay, once energized, will stay energized forever, so it is assumed there are other contacts (not relevant to the argument) in the "dotted section" of the circuit that wil l be used to de-energize Relay R. This type of requirement, where the conditions necessary to energize a relay can alter once the relay has energized, is very common. It can be looked on as registering that a certai n condition has occurred. ln this case, a button has been pushed. This sort of circuit is known as a stick circuit- the relay "picks and sticks." The front contact of Relay R used is consequently known as the "stick contact."

78

Function Names

Function Nam es A coll ection of letter codes is used to indicate the function of the items represented in circuit diagrams. The form of this designation is usually the identity of the item (i.e., usually the switch or signal number, or the track-circuit alpha reference), followed by a number of letters defin ing the function related to the item. The last letter in the name indicates what the item is, using the following letters (among others): R

Relay

E

Lamp (E lectric lamp, as distinguished from oil!)

G

Signal Mechanism

Note that "R" has other meanings when it 1s not this last letter, while "E" has the same meamng wherever it is used.

The letters preceding this last letter act li ke adjectives, giving more detail. Letters used in these positions include (not all uses are listed here- see the AREMA Signal Manual for more): C

Correspondence or Check

G

Signal (siG na l)

K

Indicating or detection (indiKating)

P

Repeater (rePeater)

S

Stick

T

Track c ircuit

W

Switch (sWitch)

CR

C heck relay

GE

Signal lamp

KE

Indicati ng lamp

TR

Track Relay

WM

Switch motor

For instance:

Further letters add more detail if required: D

Green (proceeD or D istant)

H

Yell ow (approacH or Home)

N

Normal

R

Red or reverse

79

6 - RELAYS AND RELAY LOGIC

Typical and common names encountered include: 105TR

Track relay fo r I 05 track circu it

105 TKE

Track indication lamp fo r 105 track circuit

23 RGE

Red lamp of 23 signal

23 HE

Yellow lamp of 23 signal

345 NWPR

Norma l repeater relay for 345 switch

345 RWPR

Reverse repeater relay for 345 switch

345 RWCE

Reverse correspondence indi cati ng lamp for 345 switch

Although the letter "R" represents relay, red, or reverse, there is usually no confusion. In the examples above, the letter "R" is associated w ith either a "G" for signal (inferring red) or a "W" for switch (inferring reverse), or is the last letter (for relay). In any case, these very common names become familiar with experience.

Example I developed the circuit for a repeater re lay. The letter "P" is used in the name to indicate such a function: 105 TR

Track relay for I 05 track circuit

105 TPR

Repeater relay for the track relay of l 05 track circuit, usually termed track repeater relay

The examples above are a small selection of the letters used. The letter codes and resultant names are called nomenclature, and are described in more detail in the A REMA Signal Manual.

Practical Circuits 2-Aspect Signal Consider a simple 2-aspect signa l showing either red or green. It may be considered to be a 2-state device (i.e., red or green). A relay is also a 2-state device--energized or de-energized. Then, one relay is sufficient to control one 2-aspect signal. However, a decision has to be made about "which way up" to use the relay (i.e., energized to light the green, or energized to light the red). Any fai lures will result in the controlling relay being de-energized, so to be safe, the relay must be de-energized when lighting the red lamp, and consequently energized to light the green. As a result, the relay may be said to be contro lling the green aspect, and can be named DR as a result (note it is not DGR for some reason) . It is of no concern for thi s exercise what controls there are in the DR circuit. The circuit can be drawn (where the * represents the signa l number):

80

Relay Logic and Equivalents

*OGE

B

*DR

N

*DR

(Please note that these circuits are being discussed to show the logic and the use of symbols, not the true and fu ll detail of the circuit.) 3-Aspect Signal A 3-aspect signal needs two controlling relays. The first contro ls the ye llow aspect, and so is called the HR and the second controls the green aspect, so again is called the DR. When the signal is required to be showing red, both relays are de-energized . If the HR is energized, the signal shows yellow, and then if the DR is also energized, the signal shows green. To ensure the correct sequence, the DR cannot become energized unless the HR is already energized.

*HR *HR

t

•

*DR

*OGE

B

*HR

*DR

N

f"\..J *H GE

*RGE

The HR decides whether the signal shows red or one of the that aspect is.

PROCEED

aspects, and the DR decides which

Relay Logic and Equivalents The examples above developed relay c ircuits from statements of the desired operation: Relay X to be energized only when Relay A AND Relay B are both energized. These statements are, in fact, the basis of the form of the expressions used in programming vital microprocessors. Using the ALSTOM VP!®vita l microprocessor interlocking system as an example, the above expression translates to the expression:

81

6 - RELAYS AND RELAY LOGIC

BOOL X = (A * B) where the asterisk (*) is the symbol for "AND," and "BOOe' indicates to the compiler reading the expression that this is a logic statement. This is a form of Boolean logic, and those who have had exposure to mathematical logic will have seen this before. One of the early developers of this logic, and the originator of much of the terminology, was George Boole (1815-1864). His name is used for the type of logic, and in the VPI expression (a lthough it is not spelled correctly!). Other symbol systems are used in Boolean logic, but many include symbols not readily available on a keyboard. The symbols used for the ALSTOM VPT are: * equals and,· + equa ls or (there are mathematical reasons for these choices); and .N. equals negation (the equivalent of a back contact). Relay circuits can be translated into logic expressions, and vice versa, as demonstrated in the examp les above. The underlying logic for relay circuits and vital microprocessors is basically the same. One of the differences is that there is essentially no limit in a microprocessor system on the number of "contacts" on a relay. This means that combining circuits, as illustrated in Example 5, is not necessary m a microprocessor system-in fact, it may not be possible, especially in vital microprocessors.

Safety of Relay Circuits With a basic understanding of relay construction and circuit logic, discussion can turn to the safety considerations of relay design and circuitry. Some of these points have been mentioned in the examples of relay circuits. Rai lroad vital signal relays are much larger and heavier than typical industrial relays of similar ratings. The signal relays are designed to be extremely reliable, but more importantly, they are designed so that fai lmes will result in a predictable state. A front contact of a vital signal relay uses a carbon block impregnated with silver as the "fixed" front, contacting a si lver heel contact. This is to assure that the front contact cannot weld even under heavy currents. An adjustable stop of nonmagnetic material is in the armature that will maintain an air gap between the armature and the pole faces of the coils. Even if this stop should fall out, fixed stop pins of nonmagnetic material are inserted into the pole faces to maintain a minimum air gap. This air gap is sufficient to assure that the armature w ill not remain attracted to the pole face due to residual magnetism when there is no current in the coil. The armature must return to the de-energized position by gravity alone, although springs can be applied for assistance. There are other requirements on the construction, such as the design of the " hinge" or support of the armature, the contact spacing, and material selections. The goal of a ll of these requirements is to assure that, under any circumstance, the front contacts wi ll not close or will not remai n closed longer than intended, when the coil is de-energized. This understanding of the vital signal relay leads to the root principle of vital relay circuits. The energized position of a relay shou ld always represent the least restrictive state of that particular function. It is always possible for a wire to break, and it is very difficult to avoid the effects of such failures. It is not nearly as difficult to make sure that there will be no false feeds (although this cannot be said to be easy by any means). Thus, vital circuits are designed to pick the relay to c lear the signal, or to unlock the switch, or to verify that the switch is in a particular position. Often this principle is expressed as the closed circuit principle; that is, the less restrictive condition is the closed circuit.

82

Safety of Relay Circuits

"CLOSED CIRCUIT PRINCIPLE: The principle of circuit design where a normally energized electric circuit which, on being interrupted or de-energized, will cause the controlled function to assume its most restrictive condition." (AREMA Signal Manual) These ptinciples are also known as fail safe, which means that any failure (such as a broken wire) w ill cause the system to revert to a condition known to be safe. The safe condition might be a signal at red, or a switch locked, or similar conditions. Unfortunately, fa il safe has become overused in other industries, and the meaning in railroad signaling has perhaps become obscured. Relays are also periodically tested to assure that they are in good repair and are operating within their specified parameters. The Federal Railroad Admin istration's instructions concerning signal and traincontrol systems (CFR 49, Rule 236) directs the frequency of testing of various relay types, as well as some of the parameters that must be tested . The other parameters are set by the relay manufacturer.

83

Chapter 7

Train Detection