page_title: Track circuit
section_title: The basic circuit
hierarchical_section_title: Track circuit / Principles and operation / The basic circuit
context_page_description: A track circuit is a simple electrical device used to detect the presence or absence of a train on rail tracks, used to inform signallers and control relevant signals.
context_section_description: A track circuit typically has power applied to each rail and a relay coil wired across them. When no train is present, the relay is energised by the current flowing from the power source through the rails. When a train is present, its axles short (shunt) the rails together; the current to the track relay coil drops, and it is de-energised. Circuits through the relay contacts therefore report whether or not the track is occupied.
  Each circuit detects a defined section of track, such as a block. These sections are separated by insulated joints, usually in both rails. To prevent one circuit from falsely powering another in the event of insulation failure, the electrical polarity is usually reversed from section to section. Circuits are powered at low voltages (1.5 to 12 V DC). The relays and the power supply are attached to opposite ends of the section to prevent broken rails from electrically isolating part of the track from the circuit. A series resistor limits the current when the track circuit is short-circuited.
caption: Schematic drawing of track circuit for unoccupied block
   (series resistor next to battery not shown)