The foundational technology of automatic railway signaling. A power source feeds a low-voltage current down one rail, through a relay at the far end of the block, and back up the other rail. When a train enters the block, its solid steel axles shunt the current away from the relay, causing the relay to drop and trigger a red signal to protect the occupied track.
The maintainer is out looking for a broken bond wire that dropped the track circuit and threw a red board at CP 42.
Heavy rust or a buildup of leaves on the railhead can insulate the wheels and fail to shunt the track circuit, making a train invisible to the dispatcher.
Related terms
Block signal
A fixed trackside light or semaphore that tells an engineer whether the track segment ahead is clear.
Interlocking
An arrangement of signals and switches interconnected to prevent conflicting train movements.
Signal aspect
The visual appearance of a railway signal that conveys a specific rule to the crew.