Raurimu Spiral
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The Raurimu Spiral is a single-track railway spiral overcoming a 132 m height difference, in the central North Island of New Zealand, on the North Island Main Trunk Railway. It is a notable feat of civil engineering.
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[edit] Background
During the construction of the central section of the main 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 west.
South of Taumarunui the terrain is steep but not unmanageable, with the exception of the stretch between Raurimu and National Park, where the land rises too steeply for a direct rail route. A direct line between these two points would rise 200 m in a distance of some 5 km, 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-km detour and nine massive viaducts. Even then, the gradient would have been over 1 in 50.
[edit] The Spiral
The problem was solved in 1898 by R. W. 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 scheme. Probably the most remarkable feature is that, even today, there is no place to view the complete line. By all accounts Holmes visualised the layout in his imagination.
The railway is laid out in an ascending spiral southwards, with two tunnels, a circle and three hairpin bends. From the north, trains pass Raurimu before executing a sharp 180° 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 towards Wellington. Two km further on the line undertakes two further sharp turns, to the right and then to the left.
After the second of these turns the train has travelled 6.8 km since passing Raurimu and has risen 132 m, although the straight line distance between the two points is a mere 2 km.
[edit] See also
[edit] References
[edit] External links
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- Mapping from Multimap or GlobalGuide or Google Maps
- Aerial image from TerraServer
- Satellite image from WikiMapia
