Track Circuits
Domain 2 tests occupancy detection: how a track circuit proves a section clear, what shunting sensitivity means, and the conditions under which a track circuit can fail to detect a train or a broken rail.
What a track circuit does
A source feeds current through the rails to a track relay at the far end; the relay is up only while the rails are unshunted. Wheels and axles short the rails, dropping the relay, which places signals at Stop, holds routes, and removes crossing and cab-signal energy. Coded circuits also carry aspect information; some configurations additionally detect broken rails.
Configurations
- DC non-coded: simplest form; relay up means clear; vulnerable to foreign and stray current.
- Coded: energy is interrupted at a code rate (for example 75, 120, or 180 pulses per minute) to convey cab signal aspects; loss of code gives the most restrictive indication.
- AC, audio frequency, and overlay: jointless designs; AF overlay uses frequencies above propulsion current and avoids insulated joints.
- Single-rail (common rail): traction return is carried in one rail; requires impedance bonds and careful bonding in electrified territory.
Design limits
Shunting sensitivity is the standard 0.06 Ω - a single car axle across the rails must drop the relay. Ballast resistance is the leakage path between rails; wet, salted, or cinder-fouled ballast can pull it low enough that the relay will not pick up on a clear track (right-side failure, but a service interruption). Conversely, high ballast resistance combined with long circuits and high rail resistance can prevent a shunt from dropping the relay, giving a false clear. Rail resistance between the shunt and the relay is why the rules require shunt tests at both ends and at the feed end of the circuit.
Worked example
A 10 V source with 2 Ω of feed resistance and a 0.5 Ω relay draws 10/2.5 = 4 A unshunted; the relay is firmly up. Apply the 0.06 Ω test shunt at the relay end: the shunt parallels the coil at 0.0536 Ω, total circuit 2.054 Ω, source current 4.87 A, and relay current = 4.87 × (0.06/0.56) = 0.52 A. If must-drop is 0.75 A, the relay drops and occupancy is correctly shown. Move the same shunt to the feed end, and rail resistance plus ballast leakage may let more than 0.75 A reach the coil - the test passes at one end and fails at the other, which is the exact false clear the test standard targets.
Traps
- Shunt testing only at the relay end.
- Assuming every track circuit detects a broken rail.
- Overlooking loss-of-shunt hazards: rust, sand, ice, light or single-axle equipment.
- Accepting a circuit that picks up sluggishly on wet ballast.