What is the purpose of a track circuit?

The primary purpose of a track circuit is to detect the presence of a train on a section of track, providing a fundamental safety function in railway signaling systems. A track circuit is an electrical circuit that uses the rails as conductors. A low-voltage electrical current is sent through one rail, travels through the wheels and axles of any train on the section of track, and returns through the other rail. At the end of the track section, a relay detects the presence or absence of the current. When a train is present, the wheels and axles short-circuit the track circuit, causing the current to drop and the relay to de-energize. This de-energized relay indicates that the track section is occupied. When no train is present, the current flows through the entire circuit, and the relay remains energized, indicating that the track section is clear. This simple on/off indication is used to control signals, switches, and other safety-critical equipment. For example, if a track circuit detects a train on a section of track, the signals protecting that section of track will be set to red (stop), preventing other trains from entering the occupied section. Track circuits also detect broken rails, as a break in the rail will interrupt the electrical circuit and cause the relay to de-energize, indicating that the track is unsafe. Therefore, track circuits provide a vital means of ensuring safe train separation and detecting track failures.