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14-522: The Raurimu Spiral is a single-track railway spiral , starting with a horseshoe curve , overcoming a 139-metre (456 ft) height difference, in the central North Island of New Zealand , on the North Island Main Trunk railway (NIMT) between Wellington and Auckland . It is a notable feat of civil engineering , having been called an "engineering masterpiece." The Institute of Professional Engineers of New Zealand has designated

28-419: 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 longer than the length of each loop it may be possible to view it looping above itself. The term "loop"

42-576: A zig-zag alignment at which trains must reverse direction to continue is a reversing station . One of the best examples is the Darjeeling Himalayan Railway , a UNESCO World Heritage Site railway in India, which has six full zig zags and three spirals . 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

56-413: Is also ideal for climbing steep gradients with minimal need for tunnels and heavy earthworks. 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. 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

70-787: 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 . Replica of the Brusio Spiral Viaduct at 10°28′44″N 84°49′25″W  /  10.47900°N 84.82374°W  / 10.47900; -84.82374 on the Tren Turistico Arenal , 10 km east of Nuevo Arenal, Guanacaste. The Darjeeling Himalayan Railway originally had five or six spirals but only five in operation at any one time. The line also has six reverses or zig-zags . There are three spirals on

84-500: Is the way in which it uses natural land contours so that no viaducts are needed, and only two short tunnels. Legend has it that a locomotive engineer once engaged the emergency brakes of his train upon mistaking the light of his own Guard's Van on a nearby part of the spiral for the rear of a different train directly ahead of him. 39°7.4′S 175°23.8′E  /  39.1233°S 175.3967°E  / -39.1233; 175.3967 Spiral (railway) A railway spiral rises on

98-612: The 1,000 mm ( 3 ft  3 + 3 ⁄ 8  in ) gauge railway line from Kenya to Uganda . This railway has been superseded by the Mombasa–Nairobi Standard Gauge Railway , which has removed the need for spirals by constructing tunnels and bridges. Zig zag (railway) A railway zig 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

112-471: The Piopiotea River. The most remarkable feature is that there is no place to view the complete line. By all accounts, Holmes visualised the layout in his imagination. The railway forms an ascending spiral southwards, with two relatively short tunnels, a circle and three hairpin bends. From the north, trains pass Raurimu before going round a 200° bend to the left in a horseshoe curve , climbing above

126-451: The gradient would have been steeper than 1 in 50. The problem was solved in 1898 by a surveyor in the employ of Robert Holmes , Public Works Department engineer. He proposed a line that looped back upon itself and then spiralled around with the aid of tunnels and bridges, rising at a gradient of 1 in 52. Though costly and labour-intensive, the scheme was still cheaper than the previous plan by Browne and Turner which required 9 viaducts down

140-426: The land rises too steeply for a direct rail route. A direct line between these two points would rise 200 metres (660 ft) in a distance of some 5 kilometres (3.1 mi), a gradient of 1 in 24. The area was thoroughly surveyed during the 1880s in an attempt to find a route with a lesser grade, but the only viable possibility seemed to require a 20-kilometre (12 mi) detour and nine massive viaducts. Even then,

154-624: The line has two more sharp bends, to the right and then to the left. After the second of these bends a train has risen 132 metres (433 ft) and travelled 6.8 kilometres (4.2 mi) from Raurimu – the straight-line distance is 2 kilometres (1.2 mi). Some of the sharp curves are only 7½ chains (150 m) radius. Although spirals are relatively common in the Alps , particularly in Switzerland , they generally involve extensive tunnelling inside mountainsides. A masterly feature of Holmes' layout

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168-430: 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. Zig zags suffer from a number of limitations: 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

182-640: The spiral as a significant engineering heritage site. During the construction of the central section of the NIMT, a major obstacle arose: how to cross the steep slopes between the North Island Volcanic Plateau to the east and the valleys and gorges of the Whanganui River to the west? South of Taumarunui , the terrain is steep but not unmanageable, with the exception of the stretch between Raurimu and National Park , where

196-572: The track on which they have just travelled. Two sharp bends to the right follow, after which the line passes through two short tunnels, the Lower Spiral Tunnel (384 m) and the Upper Spiral Tunnel (96 m). Trains then complete a full circle, crossing over the Lower Spiral Tunnel through which they have just passed which is 23-metre (75 ft) below, before continuing towards Wellington. Two kilometres (1.2 mi) further on

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