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For more than 30 miles south of Taumarunui the North Island Main Trunk Railway climbs steadily toward National Park, situated on the edge of the great Waimarino Plateau. In this distance the total ascent is 2,086 ft.
But over the last 7 miles from Raurimu to National Park, there is an abrupt increase in altitude of over 700 ft. This rise is overcome by a masterly example of railway engineering known throughout the world as the Raurimu Spiral.
Laid out in the form of an ascending spiral, incorporating a complete circle, three horseshoe curves, and two short tunnels, the Raurimu Spiral artificially increases the distance between Raurimu and National Park from 3+ miles in a direct line to 7 track miles. The average gradient is thus reduced to approximately 100 ft to the mile, or 1 in 52.
John Rochfort, whose flying surveys were adopted for the route to be followed, confidently stated before the parliamentary committee of 1884 that no grade steeper than 1 in 70 would be required. However, subsequent surveys in 1886-87 by J. B. Browne and C. B. Turner, also R. A. Brown and C. B. Beere, proved that it was impossible to achieve a gradient of 1 in 70.
It was therefore decided to adopt a grade of 1 in 50, but, even so, the only practicable alternative route resulting from nine surveys was a costly 12-mile line requiring nine viaducts.
The problem was finally solved 10 years later by R. W. Holmes, a senior engineer of the Public Works Department. Holmes personally surveyed the confused and bush-covered terrain, and in 1898 produced a textbook example of a spiral line. Even today the earthworks necessary to build this spiral would be considered extremely heavy, but Holmes's solution was far shorter and much cheaper than the tortuous line laid out in 1887 by Browne and Turner.
The Raurimu Spiral is a permanent memorial to R. W. Holmes-the man who has been described as "perhaps the greatest location engineer in New Zealand's history".
Over the King Country mountain section, large New Zealand built oil-fired steam locomotives of the 4-8-4 type, classes "K" and "Ka", were employed for many years. Each of these locomotives could haul a 320-ton passenger train, or a 350-ton goods train, up the 1 in 50 gradient of the Raurimu Spiral.
Since 1965, powerful 78-ton diesel-electric locomotives of the "Da" class have been used on almost all trains over this section. These highly efficient locomotives haul 380 tons per engine up the Spiral, and do it at the same or better speed than the steam locomotives, and at much lower cost.
Two "Da" class locomotives operating in multiple unit, under the control of one crew, can haul a 760-ton goods train up the Spiral as quickly as two of the most powerful steam locomotives formerly used could haul their maximum load of 580 tons.
Taken from NZR flyer's 1970, 1972.
