Raurimu Spiral - An engineering masterpiece

The small town of Raurimu almost exactly halfway between Wellington and Auckland, on State Highway 4 , is now a tourist settlement, serving the ski-fields a bit further south. It was once a frontier town, with an extensive timber milling industry, and an important minor railway town in the days before centralisation of signalling and maintenance.

But it is the railway itself that the town is most known for. Immediately to the east is the Raurimu Spiral, a significant late 19th century engineering accomplishment, that enabled the North Island Main Trunk railway line to descend from the high plateau at National Park to the valley of the Piopiotea Stream, and thence to the Wanganui River.

Between National Park and Raurimu is only 3.5 km in a direct line, but Raurimu is over 200 m lower. John Rochfort, who made a “flying” (overview) survey of the route of the line in the 1880s, showed the line going in an more or less straight line down from National Park, about where the State Highway is now. However, grades that may be acceptable on a road create severe problems for a railway. The direct line would have resulted in a gradient of as steep as 1 in 20 (or 5%), which would have needed something like the Fell system on the Rimutaka Incline to make it work. The actual grade of the railway is 1 in 50 (2%), and this was achieved by making the railway twice as long as the direct distance. 1 in 50 is still a serious grade.

The powerful ‘Ka’ steam locomotive could pull 1000 tons on level ground, but only 350 up the spiral. Modern 3MW electric locomotives still only manage 900 tonnes. To get the extra length and the acceptable 1 in 50 grade, a more detailed survey was carried out by R W Holmes. This resulted in the line being laid out and built in the form of a spiral.

The first train over the spiral ran in late 1907, less than a year before the whole Auckland-Wellington route was completed. Travelling south from the Raurimu station site (now only a crossing loop), the line doubles back on itself through a tight horseshoe curve, and heads north. It then curves east then south through a 384m tunnel, followed by a full 360 degree circle back over itself, across the top of the tunnel. A second, short,tunnel is located on the circular section. From the end of the circle the line continues south on the same grade to National Park.

Just north of the junction of the State Highway and the access road to Raurimu is a viewpoint for the spiral,and a model of it. From this point three levels of the spiral can be seen (today vegetation obscures the actual bench of the railway formation, but the route of the line can be traced through the line of electrification masts).

The first level is the site of Raurimu station, the second follows the horseshoe curve heading northwards back past the station, and the third is on the full curve, where southbound trains again head north, out of the shorter tunnel. A fourth level may be discernible to the south east, at the top of the spiral, where the line runs on a high shelf above the Piopiotea Stream.

Getting there: Raurimu is 42 km south of Taumarunui, and 7 km north of National Park, on State Highway 4. This highway runs from just south of Te Kuiti to Wanganui, with convenient connections to Auckland and Wellington. It passes to the west of the central volcanoes, and offers a scenic alternative to the conventional route between Wellington and Auckland on State Highway 1 to the east. From Taupo and Turangi the best route is via State Highway 47 to National Park.

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