Absolute block signalling

Absolute block signalling is a British signalling block system designed to ensure the safe operation of a railway by allowing only one train to occupy a defined section of track (block) at a time.^[1] Each block section is manually controlled by a signalman, who communicates with the other block sections via telegraph. This system was used on double or multiple lines where use of each line is assigned a direction of travel before the introduction of track circuits, and is still used on lines which lack track circuiting.^{[citation needed]}

A train approaching a section is offered by a signalman to his counterpart at the next signal box. If the section is clear, the latter accepts the train, and the first signalman may clear his signals to give permission for the train to enter the section.^[2] This communication traditionally takes place by bell codes and status indications transmitted over a simple telegraph wire circuit between signalmen using a device called a block instrument, although some contemporary block working is operated wirelessly. This process is repeated for every block section a train passes through. The absolute block system does not replace the use of any other form of signalling, such as fixed signals, hand signals, or detonators – and, in fact, usually relies on fixed signals.^[3]

Prior to the introduction of block systems, time intervals were used to keep trains sufficiently far apart; typically if five minutes had passed since the first train had departed then a second train was allowed to proceed; although the driver was warned that there was a train only five minutes ahead.^[4] This was insufficient to prevent a train colliding with the rear of one that had stopped unexpectedly.^[5][6]