Railway signal
QUESTION: How do railway signals automatically change from green to red as soon as the engine crosses the signal post?
P. Arunkumar, Salem, Tamil Nadu
ANSWER 1: An electric circuit called `track circuit' is responsible for changing the signals. A portion of the track is insulated from others and a low voltage source (1.2 to 2 volts) is fed at one end of the insulator track and a relay is connected at the other end. When there is no vehicle on the track, the relay energises the track, which means there is no vehicle or any other obstruction on the track and it is clear, turning the signal to green.
When the engine passes the signal post, it occupies the insulator track and shunts the track by the axle of the engine.
As a result, the relay does not get sufficient current and it de-energises and the signal changes to red. This is a continuous process.
A. Abdul Gafoor
Retd Signal Inspector
Nagercoil
Tamil Nadu
ANSWER 2: `Track-circuit' is an electrical circuit, which is fixed on the railway tracks to detect the presence of a train or vehicles on the track. It indicates whether the track is occupied by a train or not.
For setting up a track circuit the portion of rails forming the circuit are isolated by providing insulation at the rail joints.
The essential requirement is that the rails should be laid on wooden or concrete sleepers so that they are electrically insulated from each other.
One end of the rail is connected to a battery through resistances and the other end is connected to a relay, which in turn is connected to the signal.
A relay is a device used for opening or closing an electrical circuit by energising or de-energising it and used for various functions such as operation of points and clearing signals.
When the track is free, energy from the battery reaches the relay, energising it and the signal shows green.
When the engine passes the signal on the track, the two rails are short-circuited through wheel and axle of the train and the relay does not get any feed from the battery. It gets de-energised, breaking the circuit connected with the signal and thereby turning the signal to red.
S.B. Parameswaran
Station Master
Tiruchi, Tamil Nadu
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