A spiral is a hill climbing technique for railways when the topography rises faster than the train can climb. It is a alternative to a Zig Zag that avoids the need for the trains to stop.

Consider a railway climbing at a gradient of 1 in 40 (2.5% or 25m per km). A 360% spiral at 350m radius will add 1100m to the forward journey and 27m to the vertical climb. Unless the topography has a suitably shaped hill, the spiral is likely to be entirely in tunnel, creating problems if steam locomotives are the form of traction employed. To reduce potential problems with smoke, there may be some advantage to easing the gradients through such tunnels.

List of spirals

• Africa
  • Kenya-Uganda there are 4 spirals on the Kenya to Uganda line.

• Australia
  • Bethrunga Spiral (topography favourable, two very short tunnels, downhill track plain, uphill track in spiral)
  • Cougal Spiral (topography fairly favourable, one short, one long tunnel, single track).

• Bulgaria
  • Avramovo (760mm gauge)

• Canada
  • Spiral Tunnels between Hector, AB, and Hector, BC, Canadian Pacific Railway

• India
  • Darjeeling Himalayan Railway has four loops of which two are double spirals, narrow gauge

• Iran
  • Numerous spirals (very mountainous, mostly entirely in tunnel, single track)

• New Zealand
  • One spiral at Raurimu, single track, suitable hill, short tunnel.

• Switzerland
  • Many spirals (very mountainous, mostly entirely in tunnel, usually double track)

• Taiwan
  • Dulishan loop is a triple spiral (two clockwise and one counter-clockwise), Alishan (阿里山) Forest Railway, narrow gauge, single track

• United Kingdom
  • Dduallt Loop is the only British spiral, Ffestiniog Railway in Wales

• United States
  • Tehachapi Loop, California
  • Georgetown Loop Rail, Colorado

See also

• Horseshoe Curve

External link

• Helixes, Tunnels, Spirals, and Other Unique Trackage