A headshunt is a short length of track provided to release locomotives at terminal platforms, or to allow shunting to take place clear of main lines.
Terminal headshunt
A 'terminal headshunt' is a short length of track that allows a locomotive to uncouple from its train, move forward, and then run back past it on a parallel track. Such headshunts are typically installed at a terminal station to allow the locomotive of an arriving train to move to the opposite end of its train, so that it can then haul the same train out of the station in the other direction.
Reversing headshunt
Found primarily on metro systems, rapid transitlight rail networks, and tramways, a 'reversing headshunt' allows certain trains or trams to change direction, even on lines with high traffic flow, whilst others continue through the station. Typically there will be two running lines, one for each direction of travel, and the headshunt will be positioned between the two running lines, linked to both by points. Although most trains will pass through the station and continue in the same direction, an individual train may be directed into the reversing headshunt, before exiting onto the other running line, in the opposite direction of travel. This procedure allows a greater frequency of trains on a city-centre section of the line, and reduced frequency on the suburban sections, by allowing certain trains to shuttle back and forth only on the city centre part, using the reversing headshunts to change direction within the flow of trains.
Shunting neck
The term headshunt may also refer to shunting neck or 'shunt spur': a short length of track laid parallel to the main line for the purpose of allowing a train to shunt back into a siding or rail yard without occupying the main running-line.
Run round
A run round loop is a track arrangement that enables a locomotive to attach to the opposite end of the train. It is commonly used to haul wagons onto a siding, or at a terminal station to prepare for a return journey. This process is known as "running round a train". Although a common procedure for passenger trains when the majority of them were locomotive-hauled, the maneuver is now becoming rarer on public service railways. Increased use of multiple unit and push-pull passenger services avoids the requirement for dedicated track and the need for railway staff to detach and reattach the locomotive at track level. However, on heritage railways run-round loops are still usually more or less necessary at each end of the running line, partly because train services are usually locomotive-hauled, and partly because the run-round operation gives added interest to visitors. This practice is still very common on Intercity services in Victoria, Australia.
If a terminal station has or no longer has a run-round loop, trains are restricted to multiple units or Top and Tail trains.
Relay engines
Sometimes a terminal station has no run-round loops, the absence of which is overcome by coupling a relay engine to the rear to power the next out bound trip. The original engine of the arriving train shunts to the relay engine siding where it awaits the arrival of the second train, and so on. Examples in steam days include Fenchurch Street and Kingsgrove.
