Spiral (railway)

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.

[1][2] 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.

[38][39] The Darjeeling Himalayan Railway originally had five or six spirals but only five in operation at any one time.

[56][57] There are three spirals on the 1,000 mm (3 ft 3+3⁄8 in) gauge railway line from Kenya to Uganda.

Spiral viaduct of the Bernina Express near Brusio, Switzerland.
Guanjiao Spiral on Qinghai–Tibet Railway at night. The route containing the spiral was replaced by a 32-km long tunnel (at the time of completion the longest in China) in 2014.
Loop (Agony Point) on the DHR, India
Okoba spiral and zig zag in Hisatsu Line , Japan
Spiral loop, west of Rainbow Bridge in Tokyo, Japan.
Spiral in Thazi – Taunggyi line, Myanmar
Geumdae 2nd Tunnel in Jungang Line , South Korea
Toua spiral tunnel on the RhB Albulabahn
Triple spiral loop on the Alishan Forest Railway
The bridge on the spiral loop at Dduallt on the Ffestiniog Railway , Wales.
Tehachapi Loop , on the Union Pacific Railroad , California, United States, viewed from the air.
1903 view of Riflesight Notch loop, near Rollins Pass in Colorado