Signalling and Communication Systems

Abstract

The signalling system is an essential part of a railway. The essential purpose of the railway signalling system is:

• to maintain a safe distance between following trains on the same track;

• to safeguard the movement of trains at junctions, and when crossing a path which could be taken by another;

• to regulate the passage of trains according to the service density and speed required

However, the signalling system also affects the sort of service that can be run on a section of railway. The signalling system thus has to perform in a safe manner, but perhaps more importantly, if something goes wrong it has to fail in a safe manner. When railways were first introduced as a means of transportation, there were no fixed signals and Policemen wandered around the stations changing the points at junctions and giving instructions to train drivers by coloured flags by day and oil lamps by night. There was no means of communication with adjacent stations, so there was no way of knowing whether it was safe to let a train set off for the next station. All that could be done was to give the Policeman an egg-timer and a train was allowed to leave a station a fixed time interval after the previous one. There were amazingly few accidents, probably because train speeds were not very high at that time.