Signal Basics

Signals are provided bearing in mind the following main considerations a) Safety is ensured by the indications offered by the signals b) Operational needs are satisfied.

The need to ensure safety is always the topmost consideration and under no circumstances the signalling arrangement can compromise with this primary requirement. The operational needs is next to be satisfied. thus if a signal is provided which never shows a proceed aspect while safety is ensured nothing is achieved as no train can move on the signal. Hence the need to ensure that the signaling arrangement ensures that trains can be moved effectively. Satisfying these two considerations require the definition of various types of signals and the location of these signals to result in the best signaling arrangement for a particular use. Any Signaling arrangement requires to follow a set of rules which are normally ensured by the Railway for ensuring that the trains can be moved safely and in a manner which is desired for the optimization of the use of the facilities. Any rules of operation of a Railway therefore defines a set of signals which can be generally categorized into the following types a) Main Line signals b) Signals for operation of trains within yards c) Signals for use under special circumstance and not meant to be used normally. When trains follow these signals special precautions are needed to be observed as these signals may allow entry of trains into territories which are already occupied by other vehicles or may be occupied by other vehicles. The general rules of a main line signal is that the indication of a main line signal is valid up to the next main line signal negotiated by the train. If a main line signal indicates the train to move ahead exhibiting proceed aspect as required it indicates to the train that the portion of the railway track up to the next main line signal negotiated by the train is free from any obstruction and as long the train can stop at the next signal there is no danger to the train of facing a obstruction. Clearance of main Signal means track leading to next main signal is clear of any obstruction. If we refer to figure 1 this will mean clearance when Signal S1 is cleared it means the train can run until it reaches Signal S2.

Here a very important item is the requirement that the train should be capable of stopping at the next signal. This statement should actually be read as the need of a train to ensure that it can stop before the signal beyond which obstruction is expected. This modification to the statement brings into the concept of multiple aspect signaling by use of which it is possible to indicate a train where a possible obstruction is. It is sufficient for most signaling systems to indicate the location of obstruction two signals ahead of it as this will normally ensure at least 2 Km distance is available before another obstruction is likely to be encountered by a train. This means that as long as trains are not run at frequencies such that trains are spaced 2 Kms apart this arrangement is sufficient. This in turn means that with average speeds of 60 Kmph trains are running at interval of 2 minutes. This is a very good frequency of train operation and so covers most circumstances. This in turn means that the signals need to indicate the following conditions a) No obstruction after second next signal b) Track beyond the second next signal is obstructed c) Track beyond the next signal is obstructed Hence the signals need to indicate three conditions which is typically indicated by a Green for the condition a) Yellow for the condition b) and red for the condition c).

Referring to Figure 2 with 2 trains one in rear of S1 and another in advance of S3 Signal S1 when cleared can be green but signal S2 can be yellow indicating that there is a obsutrction ahead of the next signal S3. These three conditions however are practically vitiated under certain conditions as when the train is approaching a station and it is required to take a turn away from the straight route or due to operational reasons you need to have signals near to each other than 2 Kms.

The vitiation of the conditions cannot lead to loss of considerations of safety. The solution to this is to provide signals which will satisfy conditions of safety as well as ensure that the operational requirement. Requirement of safety under this condition can simply be met by ensuring that the signal in rear which is at a distance more than the braking distance from the signal showing either stop or the setting of route which is a turn out at yellow. This solution however can reduce the speed of trains too early as the signal in rear may be far from the signal at which the train has to stop or take a turn out. Hence the introduction of a fourth aspect helps in the matter. The fourth signal is not a new color of the signal but is the use of two yellow signals on the same post and is referred to as double yellow. The double yellow signal when used can be used till the braking distance is available and can be repeated over several signals leading to the signal immediately in rear at which the train has to stop actually. The signal immediately in rear should display a yellow aspect. While here again the train can be made to slow down unnecessarily but practically this is a good enough solution for most conditions of manual train running. In figure 3 the use of signal S3 about 1Km in rear of the diversion takes care of this need. Classification of signals Main Line Signals at a station can be classified into the following: a) A signal which controls entry into and directs it to specified line of a station yard which is in many cases referred to as Home signal and can be generally called the first stop signal of a station Signal S5 is a Home Signal in Figure 3.

b) A signal which allows departure from specified lines of a station yard which is in many cases referred to as Starter Signal. In Figure 3 Signals S7 and S9 are starter signals. c) A signal which allows a train to leave a station yard which in many cases referred to as Advance Starter Signal and can be generally called the last stop signal of a station. Refer to Fig 4 where Signal S11 is a Advance Starter Signal. d) A signal which is at adequate braking distance from the from the first stop signal and often called the Distant Signal and is a approach repeater signal. In figure 3 Signal S3 is the distant signal.

The first stop signal is required to be placed so as to protect the trains in the station and trains coming into the stations as well. It is therefore necessary that the signal is placed before any train stabled at the station. The best location of placement of this signal is therefore just in front of all connections of the station. To ensure that trains approaching the stations do not overshoot and lead to collision the first stop signal is normally placed sufficiently before the first junction. In Railway parlance the first stop signal should be placed at a distance equal to the Block overlap for the signaling system in rear of the first facing point. This will ensure that the first stop signal will not only protect all trains in a station and will also allow for some level of failure of the drivers to stop well behind the obstruction. To understand the implication of this if we consider Fig 3 the signal S5 is the first stop signal. The point P1 is the location of obstruction to a train approaching signal S5. When signal S5 is at Red even if the signal is overshot the point the train will be expected stop before this point if there is enough distance between Signal S5 and P1. Such distances beyond a signal are refereed to as overlaps and depending on the policies of Railway operations can vary.

In conventional Railway operations two types of overlaps are identified a) Block overlap which is the distance required between the first stop signal in a station and a point of potential obstruction. This is the overlap required when approaching signal whose authority of control is with different authorities. b) Signal overlap which is the distance between a signal and a point of potential obstruction beyond it when the signal from which it is being approached it is with the same authority. The block overlap is obviously longer than the signal overlap as it is expected that the when the control is with the same authority the chances of keeping an obstruction beyond a signal to which a train has been allowed to come will be less. The concept of block overlap and signal overlap was more relevant in older signaling environment as in those earlier times when the technology was not well developed different authorities controlled different stations and communication facility between them was limited. This meant there was a more chances that a train may be dispatched from a station not aware of obstruction existing at the next station being approached than when a signal lead to a signal within the control of the same authority. In fig 3 the signal S5 is placed an adequate distance in rear of the point P1 so as to ensure that in the unlikely event of a train overshooting signal S5 the point will not be infringed thus saving the train from being derailed due to a point not being in the proper position to allow trains to run over it. Distance between Point P1 and S5 should be more than the block overlap.

With the improved highly reliable telecommunication infrastructure to support the Railway signaling the separate concept of Block overlap and signal overlap is significantly diluted. In fact what is more relevant is the assessment of actual overlap requirement based on the speed of the trains and its braking capabilities. In the present scenario the use of additional signals to improve safety under this condition is made available and strictly speaking the only overlap which needs to be considered is the signal overlap based primarily on the braking distance. After the first stop signal other signals are provided to ensure movement in the station area as required. signals are required to ensure that all diversions are protected properly. This means that if there is a diversion protection is available. Signal S5 in figure 3 protects the diversion provided at point P1. Protection while a train negotiates a diversion can be ensured by two methods a) By a signal placed just ahead of the diversion within say not more than 180m typically. b) By locking the switches allowing the diversion till movement leading from the signal leading to the diversion and in this condition the signal can be located any distance before the diversion or even before other diversions Both methods are actually same and is done by locking the movement of the switches till train signaled to move over the point has passed over it. But in terms of signaling the two are slightly different due to the fact In case a) the signal is very near to the switch and so when the signal is cleared the train is over the point almost immediately and simply by ensuring that the point zone track circuit is dropped it can be ensured that the switch cannot be moved. This is covered b what is referred to as Approach locking. In case b) however the signal leading to the diversion can be considerable distance away and so will take considerable time to hit the point zone track circuit. Thus another more

elaborate locking is involved which proves sequential operation of tack circuits to determine whether a train which has been signaled has completed movement over the diversion and till such time it is completed the locking of the switch needs to be maintained. This is referred to as Route Locking. If looked at from a fundamental point of view there is very little difference between a) and b) and b) is actually an extension of a).

Stations will have signals which allows trains to depart from a station when it is desired to do so. To signal a train to despatech from a station signals are located at the exit point of each line on which trains wait at stations. Signals S7 and S9 in figure 4 shows such signals. These signals are referred to as Starter signals and allows trains to depart from the tracks behind them. The tracks from which the starter signals allow departure are referred to normally as berthing tracks. Quite ogten a signal ahead of the starter signals are placed ahead of all the converging tracks as S11 in figure S11. This signal is referred to as Advance starter signal and allows better management of trains within the stations.

In bigger stations facilities for adding or stabling of coaches or wagons are provided. These are by the provision of short lenght of lines connected to one of the berthing tracks and these are known as sidings. In other instances there may be need of attaching coach or locomotive of a train on one berthing track and add to a train on another berthing track. The availability of advance starter signal is for facilitating such movements. For such movements a seperate type of signal is used refered to as shunt signals. A typical arrangement of shunt signals for movement form siding is shown in Fig 5.

Signal S101 and S102 are shunt signals allowing shunt movements from and to the siding as shown

Train operations require signals other than main line signals to control movement of trains within yard for management of train operations. Such movements are of two types a) Movements required for changing coaches, locomotives, wagons and maintenance activities b) To take care of failures of the main signals Movements as described in a) are taken care of by signals referred to as shunt signals Movements as described in b) are taken care of by signals referred to as calling on signals/indicators. Shunt signals are generally placed after the first stop signals and before the last stop signals at a station. This ensures that shunting can take place within the limits of a station and trains moving in form other stations are protected from conflicting with shunt

movements which is an entirely local operation. Unlike main signals shunt signals can be cleared even when a track is occupied in the route of the shunt signal. Shunt signals are often placed on starter signals and just in front of the diverging junction facing the starter signals. A typical arrangement of shunt signals is at Figure 6. Shunt signals are represented as small circles in the figure.

Shunt signals can be independent or dependant. When a signal is co located with a main signal it is a dependant signal and may be only referred to as main signal no with S added at the end. Thus shunt signals on Signal S7 and S9 are S7S and S9S and are dependant signals. Shunt signal S102 is a independent shunt signal.

When designing signal plans symbols used are as listed below:

When showing a signal normally circles are drawn depicting each aspect indicated by a signal. If a signal has two aspects two circles are shown, if it has three aspects then three circles are drawn. In side the circle lines drawn indicate the color of the aspect. A vertical line indicates red aspect, horizontal line indicates a green aspect while a line at 45 deg usually drawn left to right indicates a yellow aspect. All signals display an apsect normally. For all stop signals this is red. The aspect which is normally displayed by a signal is indicated by drawing the line twice within the circle. Using these symbols a typical signalling plan will look as below