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Raurimu SpiralThe Raurimu Spiral is a notable feat of engineering in the central North Island of New Zealand. During the contruction of the North Island Main Trunk railway line between Wellington and Auckland, a major obstacle was faced - 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 east. For the part of the course of the railway south of Taumarunui, the line is steep but not unmanageable, with the exception of a stretch between Raurimu and National Park, where the land rises too steeply for normal rail services. A track between these two points would rise 200 metres in a distance of some five kilometres, a slope of 1 in 24. Land surrounding the area was thoroughly surveyed during the 1880s, in an attempt to find an alternative route that would allow for a shallower slope, but only one possibility allowed for a viable line, and that required a 20 kilometre detour and nine massive viaducts. Even then, the slope would have been over 1 in 50. The problem was finally solved in 1898 by R. W. Holmes, Public Works Department engineer. Holmes proposed a line that looped back upon itself and then, with the aid of tunnels and bridges, spiralled around, rising at a gradient of 1 in 52. Though costly and labour intensive, the scheme was still cheaper than the previous scheme. The railway is laid out in an ascending spiral, with two tunnels, a circle, and three hairpin bends. Entering from the north, trains pass Raurimu station before executing a sharp 180-degree turn to the left, climbing above the track they have just travelled. two sharp turns to the right follow, after which the line passes through two short tunnels. After these, the line completes a full circle, crossing over the longer of the two tunnels through which the train has just passed before continuing on towards Wellington. Two kilometres further on, the line undertakes two further sharp turns, firstly to the right and then to the left. After the second of these turns the train has travelled 6800 metres since passing Raurimu station and has risen 132 metres, although the straight line distance between the two points is a mere two kilometres. External links
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