Spiral (railway)

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For the engineering term applying to railway track geometry, see Track transition curve.
Spiral viaduct near Brusio, Switzerland.

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 of sufficient length, it is often possible to view the train looping above itself.

A spiral loop should not be confused with the transition spiral or spiral easement used to provide a transition from a tangent into a horizontal circular curve. Spiral easement is used to avoid abrupt changes in the sideward acceleration experienced by a railway vehicle and the passengers in the vehicle approaching the horizontal circular curve and to prevent abrupt forces and discomfort. These curve layouts are also used in highway engineering.

Spirals are not generally needed on tramways (street running or reserved track) or light rail lines because trams traditionally have all wheels powered, giving the ability to climb much steeper direct gradients than railways. In addition, the space consumed by a spiral is usually not available in an urban environment.

A similar feature to railway spirals in road design is the pigtail bridge.


On a railway climbing at a gradient of 1 in 40 (2.5%, or 25 m per km) a 360-degree spiral at 350 m radius will add 2200 m to the forward journey and 55 m to the vertical climb. Unless the topography has a suitably shaped hill, the spiral is likely to include a tunnel, increasing construction costs and creating problems if steam locomotives are employed. If a convenient side valley is available, then a horseshoe curve may be possible.

The spiral needs to climb about 6 m in order to bridge itself. With steam locomotives and to a lesser extent with diesel locomotives, the gradient in any long tunnel(s) should be less than the ruling grade to avoid problems with fumes and dampness causing the driving wheels to slip.

List of spirals[edit]



  • Bethungra Spiral two very short tunnels, uphill track in spiral at 1 in 66 gradient, downhill track plain at 1 in 40 gradient
  • Cougal Spiral one short and one long tunnel, single track






People's Republic of China[edit]

  • Yangjiawan – Qinling Tunnel
  • On the Chengdu-Kunming Railway, there is a spiral north of Heijing Town





Loop (Agony Point) on the DHR, India


  • Numerous spirals through very mountainous regions, mostly entirely in tunnel and single track, e.g. the Three Golden Lines.




Okoba spiral and Zig zag in Hisatsu Line, Japan


  • KenyaUganda four spirals on the Kenya to Uganda line.

Mazeras Spiral near Mombasa, at: -3.995257° 39.546742° (map)
Spiral near Makutano station, at: -0.043457° 35.641461°
Spiral near Equator, Kenya station, at: -0.007183° 35.551033°

South Korea[edit]

Geumdae 2nd Tunnel in Jungang Line, South Korea
  • Geumdae 2nd Tunnel – between Geumgyo station and Chiak station on Jungang Line – one loop, single track. New double track tunnel is under construction to replace loop tunnel.
  • Daegang Tunnel – between Danseong station and Jungnyeong station on the Jungang Line – one loop, single track.
  • Hambaek 1st Tunnel – between Hambaek station and Jodong station on the Hambaek Line – one loop, single track.
  • Solan Tunnel – between East Baeksan station and Dogye station on the Yeongdong Line – one loop, single track but double track is installed on the middle of the tunnel; for both trains crossing each other. It will be opened in 2012.



Spiral in Thazi – Taunggyi line, Myanmar

New Zealand[edit]





  • Telgart Loop (Telgártska slučka – Slovak language), consists of 2 viaducts and 1,2 km tunnel.

South Africa[edit]


Sri Lanka[edit]


Toua spiral tunnel on the RhB Albulabahn



See Kenya above.

United Kingdom[edit]

The bridge on the spiral loop at Dduallt on the Ffestiniog Railway, Wales.

United States[edit]

Tehachapi Loop, on the Union Pacific Railroad, California, US, viewed from the air.
1903 view of Riflesight Notch loop, near Rollins Pass in Colorado

See also[edit]


  1. ^ "FEATS IN RAILWAY ENGINEERING.". The Darling Downs Gazette and General Advertiser (Toowoomba, Qld.: National Library of Australia). 25 August 1868. p. 4. Retrieved 7 January 2013. 
  2. ^ Trinity Loop
  3. ^ "TRANS-ANDINE RAILWAY.". The West Australian (Perth: National Library of Australia). 30 November 1909. p. 5. Retrieved 23 March 2013. 
  4. ^ [1]
  5. ^ fr:Ligne transcévenole
  6. ^ irfca.org
  7. ^ [2]
  8. ^ "THE SIMPLON TUNNEL.". The Capricornian (Rockhampton, Qld: National Library of Australia). 15 October 1904. p. 21. Retrieved 19 March 2013. 
  9. ^ [3]
  10. ^ Can be clearly seen on Google Maps
  11. ^ Childers, Erskine (1900). In the Ranks of the C. I. V. London: Smith, Elder. p. 86. OCLC 16633317. 

External links[edit]