What is a Track Circuit?

What is a Track Circuit? How signaling systems detect trains using electrical currents. Explained: Broken rail detection and block occupancy.

What is a Track Circuit?
November 30, 2025 9:27 am
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A Track Circuit is the fundamental electrical device used in railway signaling to detect the presence of a train on a specific section of track. It relies on the principle that the train’s steel wheels and axle connect the two rails, creating an electrical short circuit.

How Track Circuits Work (The Fail-Safe Principle)

The system is designed on a “Closed Loop” principle to ensure safety.

  • Clear Track: A low-voltage current runs through one rail, goes through a relay coil at the end, and returns via the other rail. The relay is energized (Green Light).
  • Occupied Track: When a train enters the section, its wheels short-circuit the current. The electricity flows through the wheels instead of the relay. The relay de-energizes (drops), turning the signal to Red.

Track Circuit vs. Axle Counters

Modern railways are slowly moving towards Axle Counters, but Track Circuits have a unique advantage.

FeatureTrack CircuitAxle Counter
Detection MethodElectrical Short (Continuous)Counting wheels In/Out (Discrete)
Broken Rail DetectionYES (Circuit breaks, signal turns Red)NO (Cannot detect broken rails)
MaintenanceHigh (Insulated joints needed)Low (Mounted on rail web)
VulnerabilitySensitive to rust, wet leaves, flooding.More robust in bad weather.

FAQ: Frequently Asked Questions

Q: Why do signals turn red if the power fails?
A: This is the “Fail-Safe” design. A track circuit requires power to show Green. If power is cut or a wire breaks, gravity drops the relay, automatically showing a Red signal to stop trains.

Q: Can rusty rails affect track circuits?
A: Yes. Rust is an insulator. If the rails are rarely used, the rust may prevent the wheels from making good electrical contact, potentially making the train “invisible” to the signalman.