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A siding, in rail terminology, is a low-speed track section distinct from a running line or through route such as a main line or branch line or spur. It may connect to through track or to other sidings at either end. Sidings often have lighter rails, meant for lower speed or less heavy traffic, and few, if any, signals. Sidings connected at both ends to a running line are commonly known as loops; otherwise they are known as single-ended sidings or dead end sidings, or (if short) stubs.
Sidings may be used for marshalling, stabling, storing, loading and unloading vehicles.
Common sidings store stationary rolling stock, especially for loading and unloading. Industrial sidings go to factories, mines, quarries, wharves, warehouses, some of them are essentially links to industrial railways. Such sidings can sometimes be found at stations for public use; in American usage these are referred to as team tracks (after the use of teams of horses to pull wagons to and from them). Sidings may also hold maintenance of way equipment or other equipment, allowing trains to pass, or store helper engines between runs.
Some sidings have very occasional use, having been built, for example, to service an industry, a railway yard or a stub of a disused railway that has since closed. It is not uncommon for an infrequently-used siding to fall into disrepair.
A particular form of siding is the passing siding (international) or passing loop (U.K.). This is a section of track parallel to a through line and connected to it at both ends by switches (points in international usage).
Though a siding on a class one railroad in Canada is primarily used for the meeting and passing of trains, the aforementioned term “Passing Siding” is not part of the official Canadian railroad terminology. The Canadian Rail Operating Rules edition effective May 28 2008 uses the single word “siding” and defines such as “A track adjacent and connected to the main track which is so designated in the Time Table, GBO or Operating Bulletin.” Interestingly, no mention is made of the number of switch connections to the main track such a siding may have.
The employee time table for these Canadian railroads also uses the single word “Siding” to describe this section of track. A designated siding in the railroad’s employee timetable is shown with a siding capacity which is its usable length as measured in feet. The type of siding may also be in evidence, such as “Signalled Siding” or “Non-signalled Siding” so that all employees are aware of the rules that will apply to that piece of track.
This same naming convention for sidings is followed in the American rules for railroad operation. The General Code of Operating Rules Seventh Edition effective April 1 2015 defines a siding as “A track connected to the main track and used for meeting or passing trains. Location of sidings are shown in the timetable.” The definition is interesting in that it appears to attach a single purpose to a siding and makes no mention of other possibilities such as the storage of rail cars or equipment on a siding. Like their Canadian counterparts, there also is no assigned number of main track switch connections in this definition of a siding.
Sidings allow trains travelling in opposite directions to pass, and for fast, high priority trains to pass slower or lower priority trains going the same direction. Sidings are very important for operating efficiency on single track lines, and add to the capacity of other lines. The distance between designated sidings may vary from railroad to railroad. The actual length of a railroad's designated sidings may also vary widely. The distance between sidings is a huge factor in train delay. The further a train has to travel to meet another train, the larger the delay for the stopped train. Siding capacity has always been an issue. The development of evermore powerful locomotives allowed the building of longer trains handling more tonnage. Such trains may be overlength for the present designed siding capacity.  Being cognizant of both a territory's siding capacity and the train length of all trains operating over a territory, is a very important part of a rail traffic controller's job. It may be necessary for trains to be held back at sidings which can contain them. If two trains were to meet at a siding in which neither train could fit, the process and ensuing delay can be quite onerous.
Single-ended (or dead-end) siding with similar purpose to passing loop.
- Jackson (2006), p. 192.
- Ellis (2006), p. 207.
- Jackson (2006), p. 87.
- Jackson (2006), p. 337.
- Ellis (2006), p 324.
- Jackson, Alan A. (2006). The Railway Dictionary, 4th ed., Sutton Publishing, Stroud. ISBN 0-7509-4218-5.
- Ellis, Iain (2006). Ellis' British Railway Engineering Encyclopaedia. Lulu.com. ISBN 978-1-8472-8643-7.
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