A Type C track circuit is an advanced AC track circuit designed to provide reliable train detection and strong broken rail detection, especially in electrified railway territory with AC traction return current.
The main feature of a Type C track circuit is the use of two separate AC frequencies transmitted through the rails at the same time.
One frequency is used mainly for train detection. When a train enters the block, the wheels and axles shunt the rails and change the received signal used for occupancy detection.
The second frequency is used mainly for broken rail detection. A break in either rail changes the impedance of the circuit and causes a detectable change in the signal level or phase of that frequency.
The receiver separates and analyzes both frequencies. It uses filters and independent detection circuits to determine whether the track is occupied or whether a broken rail condition may exist.
This dual-frequency design helps Type C track circuits resist interference from AC traction currents and provides a more complete method of monitoring track occupancy and rail integrity.