permissive working
A system allowing more than one train at a time to be on the same line in a block section, a signal section, or a dead-end platform line.
permissive working: controlled multi-train occupancy on one track
Permissive working is a signalling regime that allows a second train to enter a block section or signal section that is already occupied by another train. Under normal absolute block working, only one train is permitted in each block at any time; permissive working suspends this rule under specific conditions. The second train (or additional trains) enters at restricted speed, typically not exceeding 15 mph, and must be prepared to stop within the visible signal sighting distance. The leading train retains full authority; the following train has no rights to proceed beyond the signal governing that section.
Permissive working is most commonly used in yards, sidings, and dead-end platform lines where train movements are frequent, slow, and closely supervised. It permits efficient shunting operations and platform management without the need for additional line sections or complex multiple-aspect signalling. The practice reduces congestion and allows a single line to handle higher throughput of closely-spaced movements. It is also used on heavily-congested suburban lines where short block lengths and frequent stopping patterns make absolute block working impractical.
Application and control
Permissive working requires visual signalling, clear sighting lines, and operator vigilance. The signal protecting a permissively-worked section may display a "permissive" aspect, often a yellow or double-yellow indication, which explicitly authorizes entry at caution. Some railway systems use a dedicated permissive signal lever or electrical release to enable the practice only when track conditions and traffic density justify it. The system depends on drivers and shunters working at reduced speed and with enhanced awareness of the train ahead.
The term reflects its operational logic: the second train is given permission to proceed, not a right of way. Failure modes include drivers misinterpreting the signal and exceeding the permitted speed, or loss of visibility due to weather or rolling stock configuration. On lines where permissive working is in force, accident risk is managed through strict speed limits, clear sighting requirements, and the presence of a lookout on the leading train in many contexts.