An AC single rail track circuit is a train detection system that uses alternating current and places insulated rail joints on only one of the two running rails to define the track block.
The other rail remains continuous and provides a common electrical path for multiple track circuits. In electrified territory, the continuous rail may also serve as part of the return path for AC traction current.
In this setup, the AC power source is connected between the insulated rail and the continuous rail at one end of the block. The AC receiver is connected between the same two rails at the opposite end of the block.
When the track is clear, the AC signal flows through the insulated rail, through the receiver, and returns by way of the continuous rail. This keeps the receiver energized and indicates that the track is unoccupied.
When a train enters the block, the wheels and axles create a low-impedance shunt between the insulated rail and the continuous rail. This bypasses the receiver and causes it to de-energize, indicating occupancy.
The term single rail means that only one rail is segmented by insulated joints to create the defined electrical block for the track circuit.