Deepu Dharmarajan -
Posted
4 years ago
CH1 | THE PURPOSE OF SIGNALLING
SIGNALLING BOOK | CHAPTER 1
CONTENTS
1.Introduction
2.The Problems to be Solved
3.Basic Requirements
4.Lineside Signals
5.The Absolute Block System
6.Interlocking of Points and Signals
7.Single Lines
8.Further Developments
1. INTRODUCTION
In general, the railway traveller assumes that his journey will be safe. This high standard of safety which is taken for granted is the result of a long history of development. As human errors and deficiencies in safety systems become evident, often as a result of an accident, improvements are made which are then incorporated into new generations of equipment.
This is certainly true of railway signalling. It also appears to be a continuing process. We have not yet reached the situation where absolute safety can be assured.
It is useful to start by looking back at some of the early history of signalling development.
In the early days of railways, trains were few and speeds were low. The risk of a serious collision between two trains was minimal. Better track and more powerful locomotives allowed trains to run faster (requiring greater stopping distances). Railway traffic increased, requiring more and larger trains. The risks thus became greater and some form of control over train movements became necessary. The need for railway signalling had been identified.
2. THE PROBLEMS TO BE SOLVED
2.1 Collision with a Preceding Train
When one train follows another on to the same section of line, there is a risk that, if the first train travels more slowly or stops, the second train will run into the rear of the first. Initially, trains were separated using a system of "time interval" working, only permitting a train to leave a station when a prescribed time had elapsed after the departure of the previous train. Although this reduced the risk of collisions, a minimum safe distance between trains could not be guaranteed. However, in the absence of any proper communication between stations, it was the best that could be achieved at that time.
2.2 Conflicting Movements at Junctions
Where railway lines cross or converge, there is the risk of two trains arriving simultaneously and both attempting to enter the same portion of track. Some method of regulating the passage of trains over junctions was therefore needed. This should ensure that one train is stopped, if necessary, to give precedence to the other.
2.3 Ensuring that the Correct Route is Set
Where facing points are provided to allow a train to take alternative routes, the points must be held in the required position before the train is allowed to proceed and must not be moved until the train has completely passed over the points.
Depending on the method of point operation, it may also be necessary to set trailing (i.e. converging) points to avoid damage to them.
2.4 Control of Single Lines
Where traffic in both directions must use the same single line, trains must not be allowed to enter the single line from both ends at the same time. Although this could in theory be controlled by working to a strict timetable, problems could still arise if trains were delayed or cancelled.
3. BASIC REQUIREMENTS
We therefore have the basic requirements of any railway signalling system. The method of implementation has changed over the years but the purpose remains the same:-
To provide a means of communicating instructions to the driver (signals) to enable him to control his train safely according to the track and traffic conditions ahead.
To maintain a safe distance between following trains on the same line so that a train cannot collide with a preceding train which has stopped or is running more slowly.
To provide interlocking between points and the signals allowing trains to move over them so that conflicting movements are prevented and points are held in the required position until the train has passed over them.
To prevent opposing train movements on single lines.
All the above requirements place restrictions on train movements, but it is vital that the signalling system will allow trains to run at the frequency demanded by the timetable to meet commercial requirements. This must be done without reduction of safety below an acceptable level.
Signalling involves not only the provision of equipment but the adoption of a consistent set of operating rules and communication procedures which can be understood and implemented by all staff responsible for railway operation.
4. LINESIDE SIGNALS
It will probably be evident that the decisions regarding the movement of two or more trains over any portion of the railway can only be made by a person on the ground who has sufficient knowledge of the current traffic situation. His decision must be passed on to the driver of each train passing through his area of control.
In the early days railways employed policemen whose duties would include the display of hand signals to approaching trains. As the policemen also had many other duties, it soon became impractical for them to be correctly positioned at all times. Fixed signals of various designs, often boards of different shapes and colours, were provided. The policeman could then set these and attend to his other duties.
The simplest signals would only tell a driver whether or not he could proceed. From this evolved a standard layout of signals at most small stations; a "home" signal on the approach side controlling entry to the station and a "starting" signal protecting the section of line to the next station. Between these signals, each train would be under the direct control of the policeman. These signals could give only two indications, STOP or PROCEED. They therefore became collectively known as "stop" signals.
As line speeds increased, "distant" signals were introduced which gave advance warning of the state of stop signals ahead. A distant signal could be associated with one or more stop signals and would be positioned to give an adequate braking distance to the first stop signal. It could give a CAUTION indication to indicate the need to stop further ahead or a CLEAR indication, assuring the driver that the stop signal(s) ahead were showing a proceed indication.
With the addition of distant signals, trains were no longer restricted to a speed at which they could stop within signal sighting distance.
It is important to understand the difference between stop and distant signals. A train must never pass a stop signal at danger. A distant signal at caution can be passed but the driver must control his train ready to stop, if necessary, at a stop signal ahead.
The earliest signals were "semaphore" signals (i.e. moveable boards). To enable operation at night, these often had oil lamps added. With the advent of reliable electric lamps, the semaphore signal became unnecessary and a light signal could be used by day and night. Red is universally used as the colour for danger while green is the normal colour for proceed or clear. Initially, red was also used for the caution indication of distant signals but many railway administrations changed this to yellow so that there was no doubt that a red light always meant stop.
If necessary, stop and distant signals can be positioned at the same point along the track. Alternatively, certain types of signal can display three or more indications to act as both stop and distant signals.
5. THE ABSOLUTE BLOCK SYSTEM
Although time-interval working may seem crude, it is important to remember that nothing better was possible until some means of communication was invented. The development of the electric telegraph made the Block System possible.
On many railways, time-interval working on double track lines is still the last resort if all communication between signal boxes is lost.
5.1 Block Sections
In the Block Signalling system, the line is divided into sections, called "Block Sections". The Block Section commences at the starting signal (the last stop signal) of one signal box, and ends at the outermost home signal (the first stop signal) of the next box. With Absolute Block working, only one train is allowed in the Block Section at a time. The signalman may control movements within "Station Limits" without reference to adjacent signal boxes.
The accompanying diagram shows a block section between two signal boxes on a double track railway. To understand the method of working, we will look at the progress of a train on the up line.
Signalbox A controls entry to the block section but it is only signalman B who can see a train leaving the section, whether it is complete (usually checked by observation of the tail lamp) and who thus knows whether or not the section is clear.
Signalbox B must therefore control the working of the UP line block section. Similarly, signalbox A controls the DOWN line block section.
5.2 Block Bell
The signalmen at each end of a block section must be able to communicate with each other. Although a telephone circuit is a practical means of doing this, a bell is normally used to transmit coded messages. It consists of a push switch ("tapper") at one box, operating a single-stroke bell at the adjacent box (normally over the same pair of wires).
The use of a bell enforces the use of a standard set of codes for the various messages required to signal a train through the section and imposes a much greater discipline than a telephone, although a telephone may be provided as well, often using the same circuit as the block bell.
5.3 Block Indicator
This provides the signalman at the entrance to the section with a continuous visual indication of the state of the section, to reinforce the bell codes. It is operated by the signalman at the exit of the block section.
Early block instruments were "two position" displaying only two indications; line clear and line blocked. Later instruments display at least 3 indications. The most usual are:-
Line clear
Giving permission to the rear signalman to admit a train to the section.
Normal or Line Blocked
Refusing permission. The signalman at the entrance to the section must maintain his starting signal at danger.
Train on Line
There is a Train in the block section.
5.4 Method of Working
When signalbox A has an UP train approaching to send to box B, the signalman at A will offer it forward to box B, using the appropriate bell code (so that signalman B knows what type of train it is). If the signalman B is unable to accept the train for any reason, he will ignore A's bell, and leave the UP line block indicator at "Normal".
If he is able to accept the train, signalman B will repeat the bell code back to box A, and change the indication to "Line Clear". When signalman A sees his block repeater go to "Line Clear", then he can clear his starting signals to admit the train to the section.
When the train actually enters the section, signalman A sends the "Train Entering Section" bell code to box B. Signalman B will acknowledge this by repeating the bell code back to A, and turning the block indicator to "Train on Line".
When the train leaves the block section at B, the signalman there checks that it is complete by watching for its tail lamp. He then turns his block indicator to "Normal" again. He also sends the "Train out of Section" bell code to A, which A acknowledges by repeating it back. The system is now back to normal, ready for the next train.
On multiple track railways, a pair of block instruments as above is required for each line.
5.5 Extra Safeguards
The basic three-position block system, as described, relies on the correct sequence of operations for safety. A signalman could forget that he has a train in section and turn the indicator to "line clear", allowing a second train in. A detailed record (the train register) is kept of the actual times of train arrival and departure, and the times at which the bell signals are exchanged.
In most places, additional safeguards have been added to the basic system. An electric lock on the starting signal will prevent it being operated unless the block indicator is at line clear. Track circuit occupation may be used to set the block instruments to ''Train on Line" if the signalman forgets to do so.
Electric locking may also be used to ensure that signals are operated for one train movement only and replaced to danger before another movement is permitted to approach.
Although it is unusual for absolute block working to be installed on any new signalling installation today, there are many railways on which it is in widespread use. The block system, by ensuring that only one train may occupy a section of line at any time, maintains a safe distance between following trains.
6. INTERLOCKING OF POINTS AND SIGNALS
On all early railways, points were moved by hand levers alongside the points. They could therefore be moved independently of the signals controlling the movement of trains.
A great improvement in safety (as well as efficiency) was possible by connecting the point switches via rodding to a single central control point (the signal box). Similarly the signals could also be operated by wire from levers in the signal box. With the control of points and signals all in one place the levers could be directly interlocked with each other. This had the following benefits:-
Signals controlling conflicting routes could not be operated at the same time.
A signal could only be operated if all the points were in the correct position.
The points could not be moved while a signal reading over them was cleared.
In early signalling installations, all point and signal operation, together with any interlocking, was mechanical. Although it was a great technological advancement to be able to control a station from one place, the effort required to operate the levers restricted control of points to within about 300 metres from the signal box and signals up to about 1500 metres. At large stations, more than one signal box would often be necessary.
The possibility still existed for a signalman to set the points, clear the signal, the train to proceed and then for the signalman to replace the signal to normal. This could free the locking on the points before the train had completely passed over them. Signalmen's instructions usually required the complete train to pass over the points before the signal was replaced to danger.
7. SINGLE LINES
On most single line railways trains are infrequent. It is not normally necessary for two trains to follow each other closely in the same direction. Single lines were therefore treated in the same way as a normal block section with the important extra condition that trains could not be signalled in both directions at the same time.
To enforce this condition and also to reassure the driver that he could safely enter the single line, some form of physical token was used as authority to travel over the single line. On the simplest of systems only one token existed. This caused problems whenever the pattern of service differed from alternate trains in each direction.
If the timetable required two trains to travel over the single line in the same direction, the driver of the first train would be shown the token (or train staff as it is commonly known) to assure the driver that no other train was on the single line. His authority to enter the single line would however be a written ticket . The following train would convey the train staff. Although workable, this system would cause problems if trains did not work strictly to the timetable.
A further improvement was to provide several tokens, but to hold them locked in instruments at either end of the single line. The instruments would be electrically interlocked with each other to prevent more than one token being withdrawn at a time. The one token could however be withdrawn from either instrument.
If the single line block equipment fails, many railways employ a member of the operating personnel as a human token. The "pilotman", as he is usually known, will either travel with the train or instruct the driver to pass through the section. No other person may allow a train on to the single line. Operationally, this is the equivalent of the train staff and ticket system described earlier.
8. FURTHER DEVELOPMENTS
The main functions of the signalling system had now been defined, although they were to be continuously improved as the available technology developed. All signalling systems would be required to maintain a safe distance between trains, interlock points and signals and thus prevent conflicting movements, and provide the necessary information so that the speed of all trains can be safely controlled.
In recent years, the signal engineer has been asked to provide further facilities within the general scope of the signalling system. These include, train information to the operating staff, train information for passengers, detection of defective vehicles, identification of vehicles and the increasing automation of tasks previously carried out by humans. The technology exists to completely operate a railway without human intervention although the level of automation desirable for a particular railway is for that railway administration to decide. Factors such as cost, maintainability, reliability, staffing policy, passenger security and sometimes political considerations must be taken into account.
In many cases the final decision on the type of signalling to be provided is outside the direct control of the signal engineer. However, he should always endeavour to provide the best possible information and propose cost-effective solutions to particular problems so that the best decisions can be made.
