Zig zag (railway)

A railway zig zag or switchback, is a method of climbing steep gradients with minimal need for tunnels and heavy earthworks.[1] For a short distance (corresponding to the middle leg of the letter "Z"), the direction of travel is reversed, before the original direction is resumed.[2] Not all switchbacks come in pairs, in which case the train may need to travel backwards for a considerable distance.

Australia: the Lithgow Zig Zag
Germany: zig zag required to cross the outer dyke on the railway serving the island of Nordstrandischmoor
India: the Darjeeling Himalayan Railway, a UNESCO World Heritage site, with six full zig zags
Italy: zig zag on the Cecina-Volterra railway
Japan: Obasute Station platform sign displaying the switchback
North Korea: switchback between Tanballyŏng and Malhwiri
Switzerland: SBB A 3/5 on the turntable at Chambrelien railway station

A location on railways constructed by using a zig-zag alignment at which trains have to reverse direction in order to continue is a reversing station.[3]

One of the best examples is the Darjeeling Himalayan Railway, a UNESCO World Heritage site railway in India, that has six full zig zags and three spirals.[4]

Advantages

Zig zags tend to be cheaper to construct because the grades required are discontinuous. Civil engineers can generally find a series of shorter segments going back and forth up the side of a hill more easily and with less grading than they can a continuous grade which has to contend with the larger scale geography of the hills to be surmounted.

Disadvantages

Zig zags suffer from a number of limitations:

  • The length of trains is limited to what will fit on the shortest stub track in the zig zag. For this reason, the Lithgow Zig Zag's stubs were extended at great expense in 1908.[5] Even then, delays were such that the zig zag had eventually to be bypassed by a new route, opened two years later.
  • Reversing a locomotive-hauled train not purposely equipped for push-pull operation without first running the engine around to the rear of the train can be hazardous – although operating the train with two locomotives, one at each end (a practice known as "topping-and-tailing"), can mitigate the dangers.
  • The need to stop the train after each segment, throw the switch, and then reverse means that progress through the zig zag is slow.

Hazards

If the wagons in a freight train are marshalled poorly, with a light vehicle located between heavier ones (particularly with buffer couplings), the move on the middle road of a zig zag can cause derailment of the light wagon.[6]

Examples

References
  1. Raymond, William G. (1912). "Railway Engineering" (Google books). In Beach, Frederick Converse (ed.). The Americana: A Universal Reference Library, Comprising the Arts and Sciences, Literature, History, Biography, Geography, Commerce, Etc., of the World. 17. New York: Scientific American Compiling Department. Retrieved 3 January 2010. High mountain levels … may be tunneled … but … may be reached by one of several methods adopted to secure practical grades: (1) Zig-zag development … (2) Switchback development … (3) Spirals or loops …
  2. Raymond 1912. "Switch-back development … necessitating the use of switches at these ends and the backing of the train up alternate stretches."
  3. Jackson, Alan A. (2006). The Railway Dictionary (4th ed.). Stroud: Sutton Publishing. p. 285. ISBN 0-7509-4218-5.
  4. "Mountain Railways of India". UNESCO World Heritage Centre. Retrieved 30 April 2006.
  5. "The Zig-Zag Deviation". The Dubbo Liberal and Macquarie Advocate (NSW : 1892–1927). NSW: National Library of Australia. 5 December 1908. p. 4. Retrieved 17 February 2013.
  6. "The Railway Accident on the Zig-zag". Bathurst Free Press and Mining Journal (NSW : 1851–1904). NSW: National Library of Australia. 10 April 1895. p. 3. Retrieved 19 February 2013.
  7. "Historical and Archaeological Assessment of Proposed Cycleway, Near Thornleigh Quarry, Via De Saxe Close, Thornleigh (Berowra Valley Regional Park), N.S.W." (PDF). The construction of the railway siding and zig-zag to the quarry and also Hall’s Camp were associated with Amos & Co, who won the contract to build the section of railway from Strathfield to Hawkesbury River. Edward Higginbotham & Associates PTY LTD. March 2002. Retrieved 19 November 2017.
  8. "Bang rdsskisser SVJ/HFJ". www.ekeving.se.
  9. "Bandel 660". www.historiskt.nu. Retrieved 19 November 2020.
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