Track Circuit
A track circuit is an electrical circuit that uses the rails themselves to detect whether a section of track is occupied. A current flows through the rails between a transmitter and a receiver; when a train's wheels and axles short the rails together, the receiver drops out and the section is reported occupied.
How it works
Each section of track is electrically isolated from its neighbours by insulated rail joints or, in jointless designs, by tuned audio-frequency signals. A transmitter feeds a voltage into one end of the section and a relay or electronic receiver at the other end holds a clear indication as long as the current arrives. A train, a broken rail or a failed transmitter all interrupt the current and cause the section to show occupied, which is the fail-safe direction.
The occupancy indication is passed to the interlocking, which uses it to prove that a route is clear before a signal is cleared and to hold points locked while a train is passing over them. Coded track circuits can additionally carry cab signaling information to the train.
Why it matters for security
Track circuits are analogue, fail-safe devices and are not directly hackable. Their security relevance lies in where their outputs go: through relay rooms, object controllers or digital interlocking interfaces that increasingly run over IP. An attacker who can manipulate the occupancy data at that boundary can make an occupied section appear clear, or flood the interlocking with false occupancies to disrupt service.
Monitoring the digital representation of track-circuit states, and correlating it with train movements, is one of the ways operators detect tampering that the electrical circuit itself cannot reveal.
Related solution
Protecting interlocking systems at the core of rail safety
Watching how train detection data reaches the interlocking, and flagging states that do not match real movements.
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