pass

A pass occurs when two trains travelling in the same direction on a single-track railway must cross paths, and the slower or lower-priority train is diverted into a siding to allow the faster or higher-priority train to proceed. The train taking the siding stops or slows while the other train overtakes it on the main line, after which the diverted train re-enters the main track and continues its journey.

Pass operations depend entirely on the placement and configuration of sidings. A standard siding must be long enough to accommodate the entire length of the train being held, plus safe clearance margins. If a siding is too short, a partial pass becomes necessary, where the nose of the held train remains on the main line while the body occupies the siding. The signaller or dispatcher must verify siding capacity before authorising the manoeuvre.

Timing and Priority

Priority for a pass is determined by timetable, train class, and operational urgency. Express passenger trains typically pass freight or local services. A train already in the siding loses its slot and accrues delay; the passing train maintains schedule. On lightly trafficked lines, passes may occur several times daily. On busier routes, multiple sidings allow trains to pass without stopping, but single-siding sections become bottlenecks during peak hours.

The term "pass" distinguishes this procedure from a "crossing", where trains meet head-on from opposite directions. A pass involves unidirectional traffic and therefore requires only one siding to execute; a crossing, or more precisely a "meet", requires sidings or loops at both locations. Pass operations were essential to railway economics before the era of double-track main lines, and remain critical on branch lines and heritage railways today.