Australia: the Lithgow Zig ZagGermany: zig zag required to cross the outer dyke on the railway serving the island of NordstrandischmoorIndia: the Darjeeling Himalayan Railway, a UNESCOWorld Heritage Site, with six full zig zagsItaly: zig zag on the Cecina-Volterra railwayJapan: Obasute Station platform sign displaying the switchbackNorth Korea: switchback between Tanballyŏng and MalhwiriSwitzerland: SBB A 3/5 on the turntable at Chambrelien railway station
A railwayzig zag or switchback is a railway operation in which a train is required to switch its direction of travel in order to continue its journey. While this may be required purely from an operations standpoint, it is also ideal for 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] Some switchbacks do not come in pairs, and the train may then need to travel backwards for a considerable distance.
A location on railways constructed by using a zig-zag alignment at which trains must reverse direction to continue is a reversing station.[3]
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 must 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.
Passenger cars with transverse seating force riders to travel in reverse for at least part of the journey, though this issue is largely solved by longitudinal seating on cars serving such routes.[6]
Hazards
If the wagons in a freight train are marshaled 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.[7]
Dubí - local railway between Moldava and Most (Most–Moldava railway), trains have to change direction in station Dubí in order to continue further. Only one halt of a 'Z' is placed
Lemvig – Small side track from the harbor to the railway station, used only on special occasions. In reality only half a 'Z' as only one reversal is needed.
Darjeeling Himalayan Railway has six full zig zags and 3 spirals, most are from the construction of the current railway but one was added in the 1940s and at least one other was used temporarily following storm damage
Western Line, Auckland. Service runs from Britomart to Newmarket before reversing to run to Swanson. The reverse could be avoided but this would bypass Newmarket which is a major station.
Tierkrans Switchback Railway, between Barkley East station and Aliwal-North station. For economic reasons regular service was finally discontinued in 1991. Railway enthusiasts also know the line for the famous set of eight reverses.
^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. Vol. 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 …
^Raymond 1912. "Switch-back development … necessitating the use of switches at these ends and the backing of the train up alternate stretches."
^Jackson, Alan A. (2006). The Railway Dictionary (4th ed.). Stroud: Sutton Publishing. p. 285. ISBN0-7509-4218-5.
^ abVielbaum, Walt; Hoffman, Philip; Ute, Grant; Townley, Robert (2005). San Francisco's Market Street Railway. Arcadia Publishing. pp. 86–87. ISBN9780738529677.
