The heart of the signaling system is the interlocking plant. This can
be claimed as the decision making part of the system. The signal outputs
are based on the track occupancy, motor point status, output of the remote
end signal and the input from traffic controller.
This plant ensures that before a signal goes in to 'clear' (green)
state, it is absolutely safe for a train to enter into the track segment.
The traffic controllers commands are not executed if it is not safe to do so.
interlocking plant is built out of electromechanical relays. We will discuss the
type of relays under the respective section. Now we will move on to the
first element: Track Circuit; which is used to sense the presence of a train on
a track segment..