Dynamic passing loop

A passing loop (UK usage) or passing siding (North America) (also called a crossing loop, crossing place or, colloquially, a hole) is a place on a single line railway or tramway, often located at a station, where trains or trams travelling in opposite directions can pass each other.Trains/trams going in the same direction can also overtake, providing that the signalling arrangement allows it. A passing loop is double-ended and connected to the main track at both ends, though a dead end siding known as a refuge siding, which is much less convenient, can be used. A similar arrangement is used on the gauntlet track of cable railways and funiculars, and in passing places on single-track roads.

Ideally, the loop should be longer than all trains needing to cross at that point. If one train is too long for the loop it must wait for the opposing train to enter the loop before proceeding, taking a few minutes. Ideally, the shorter train should arrive first and leave second. If both trains are too long for the loop, time-consuming "see-sawing" (or "double saw-by") operations are required for the trains to cross.

On railway systems that use platforms, especially high-level platforms, for passengers to board and disembark from trains, the platforms may be provided on both the main and loop tracks or possibly on only one of them.

The main line has straight track, while the loop line has low-speed turnouts at either end. If the station has only one platform, then it is usually located on the main line.

If passenger trains are relatively few in number, and the likelihood of two passenger trains crossing each other low, the platform on the loop line may be omitted.

The through road has straight track, while the platform road has low-speed turnouts at either end.

A possible advantage of this layout is that trains scheduled to pass straight through the station can do so uninterrupted; they do not have to reduce their speed to pass through the curve. This layout is mostly used at local stations where many passenger trains do not stop.

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