Automatic Block Signaling, also called ABS, is a railway signaling system that automatically controls wayside signal aspects based on track occupancy ahead of a train.
Unlike manual block signaling, which relies on communication between block operators, ABS uses electrical track circuits and signal equipment to detect trains and automatically provide safe train separation.
In ABS territory, the railroad is divided into a series of blocks. These blocks are usually defined by the locations of wayside signals and protected by track circuits.
A track circuit is an electrical circuit that uses the rails as part of the circuit. When no train is present, the circuit remains complete and the signaling equipment recognizes the block as clear.
When a train enters a block, the wheels and axles electrically connect the two rails together. This is called shunting the track circuit. The shunt changes the circuit condition and causes the signaling system to recognize the block as occupied.
Once the block is detected as occupied, the ABS system automatically changes the signal aspects for trains approaching that block. These signals display more restrictive indications to keep following trains safely separated.
A following train approaching an occupied block may encounter a red signal requiring it to stop, or a yellow approach signal requiring the crew to be prepared to stop at the next signal.
After the leading train clears the block and the track circuit is no longer shunted, the signaling system can automatically return the signals to more permissive aspects, provided the blocks ahead are clear.
ABS improves safety and line capacity by providing continuous, automatic train separation without requiring constant human intervention from block operators.
The safe operation of ABS depends on reliable track circuits, properly functioning signal equipment, and correct maintenance and testing by qualified signal personnel.