AC single rail track circuits are different from AC double rail track circuits because only one of the two running rails is insulated at the track circuit boundaries. The other rail remains electrically continuous through multiple track circuit blocks.
The continuous rail is commonly used as the return path for AC traction power in electrified territory. It may also serve as the return path for the AC track circuit current.
In a typical AC single rail track circuit, 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.
When the track is clear, the AC signal travels 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 section is clear.
When a train enters the block, the wheels and axles shunt the insulated rail to the continuous rail. This creates a low-impedance path that bypasses the receiver, causing the receiver to de-energize and indicate occupancy.
One advantage of AC single rail track circuits is that they require fewer insulated rail joints than double rail circuits. This can reduce installation and maintenance costs.
One disadvantage is that a single rail circuit may only detect breaks in the insulated rail. A break in the continuous rail may not be detected by the track circuit, so additional safety measures may be required.