What is a Track Circuit and what does it detect?

• A. The presence or absence of a train on a track segment by measuring current in the track circuit.
• B. The speed of a train.
• C. The position of signals on the line.
• D. The presence or absence of a train on a track section by measuring current/track occupancy.

Answer: (D)

Track circuits are about occupancy: they use the electrical current in the rails to tell whether a train is present on a specific track section. A section is energized, and when a train sits on it, the wheels and axles provide a conductive path between the rails, allowing current to flow and a detector relay to indicate the section is occupied. When the train leaves, the circuit opens and the section is reported unoccupied. This is how the system tracks presence or absence of a train, not its speed or the exact position of signals.