sumitsingh wrote:The auto Signals are controlled by some mechanism may be I don't know but they have some equipments installed on the tracks.
Kindly see the attachments.
It works like this: Think of 1 or 2 km of track inserted in the middle of the drawing - the whole block.
The battery on the left sends electrons down one rail through the whole block to the other end. There they run through the coils of a relay. From there, they return via the other rail to the battery. Where there is no train, works as described. The relay closes the connection for the green light at the signal.
The tracks seen here are insulated from the rest of the network.
As soon as a single axle is on this insulated part of these rails, it form a short circuit, because it is made of metal. Now the electrons travel from one track to the other via the axle and nothing reaches the relay. This means that the switch is no longer closed for the green light, if "falls off" and closes the connection for the red light.
All this only works if the axle or axles are good enough electric conductors but with an whole train in the block, there will always be enough connection between the tracks. But I have heard of regulations to ensure that tracks don't get rusty, so very rusty track or very dirty track could pose a problem.
Bonus: If rail breaks from frost or is damaged by rocks or tampering, the circuit is broken, too and red light is shown. If the track is flooded, the short circuit is there as if there was a train. With snow and ice on the track, there can be a short circuit, too, again showing red, though in this case, it may be safe enough to proceed.