An AC single rail track circuit uses alternating current and insulated rail joints on only one of the two running rails to define the limits of a track block.
The other rail remains electrically continuous through multiple track circuit blocks. This continuous rail serves as a common return path for the AC track circuit current and may also carry AC traction return current in electrified territory.
In this arrangement, 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 along 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 occupies the block, the wheels and axles bridge the insulated rail and the continuous rail. This creates a low-impedance shunt that diverts current away from the receiver.
The drop in current or change at the receiver causes the receiver to indicate that the track section is occupied. The single rail design is different from a double rail circuit because only one rail is segmented by insulated joints.