A spiral (sometimes called a spiral loop or just loop) is a technique employed by railways to ascend steep hills.
A railway spiral rises on a steady curve until it has completed a loop, passing over itself as it gains height, allowing the railway to gain vertical elevation in a relatively short horizontal distance. It is an alternative to a zig-zag, and avoids the need for the trains to stop and reverse direction while ascending. If the train is longer than the length of each loop it may be possible to view it looping above itself.
The term "loop" is also often used for a railway that curves sharply and goes back on itself: if the railway crosses itself, then it forms a spiral or helix; otherwise, it forms the much more common horseshoe curve or bend.
^Yuan Lei (2014-12-29). "袁蕾, 吐库二线正式开通运营 南疆铁路进入电气化时代 新疆日报" [Yuan Lei, spit library second-tier official opening and operation of the Southern Xinjiang Railway electrification into the Xinjiang Daily Times (Machine generated)] (in Chinese). Archived from the original on August 22, 2016. Retrieved 2016-07-04.
^"Long Tunnels". European Tribune. 12 June 2016. Retrieved 18 November 2020.
^Devereux, Nigel (September 2017). "Costa Rican rail variety". The Railway Magazine. Mortons Media Group Ltd.: 104.
^Sigurd H. Unneberg, Sem og Slagen, en bygdebok Bind 2, Kulturhistorie, del 2 . Tønsberg: Høgskolen i Vestfold, 2001. Relevant chapter is online hereArchived 2016-03-03 at the Wayback Machine(in Norwegian)