What is a track circuit?

• A. An electrical device that measures track temperature.
• B. An electrical circuit that detects train presence on a track section by monitoring current or resistance.
• C. A signaling device for interlocking.
• D. A power supply line for overhead catenary.

Answer: (B)

Track circuits are used to know whether a section of track is occupied by monitoring the electrical characteristics of the rails. A small signal is sent through the rails and the current or impedance is measured at the other end. If the track is clear, the circuit shows a normal condition; if a train is on the section, the wheels and axles provide a conductive path that changes the current flow or resistance. That change is detected and marks the section as occupied, allowing signals to be set safely. This is different from measuring track temperature, which is for maintenance; from signaling interlocking devices, which control routes; and from the overhead power supply, which provides traction power rather than detecting train presence.