Track circuit when occupied by a train. A train’s wheels shorts out the power source applied across both running rails, in turn de-energizing a relay, which indicates that the track section is occupied.
Track circuit when occupied by a train. A train’s wheels shorts out the power source applied across both running rails, in turn de-energizing a relay, which indicates that the track section is occupied. However, the track circuit is unable to determine the exact position of the train within the track circuit section, so the block will be registered as “occupied” even if only a very small portion of it is actually occupied. Diagram from Wikipedia.