Guides To Signaling
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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.