The Raurimu Spiral is a single-track railway spiral

Spiral (railway)

A spiral is a technique employed by railways to ascend steep hills.A railway spiral rises on 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...

, starting with a horseshoe curve, overcoming a 132 m height difference, in the central North Island

North Island

The North Island is one of the two main islands of New Zealand, separated from the much less populous South Island by Cook Strait. The island is in area, making it the world's 14th-largest island...

 of New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

, on the North Island Main Trunk Railway. It is a notable feat of civil engineering, having been called an 'engineering masterpiece'. The Institute of Professional Engineers (NZ) has designated the spiral as a significant Engineering heritage site.

Background

During the construction of the central section of the North Island Main Trunk railway between Wellington and Auckland, a major obstacle was faced - how to cross the steep slopes between the North Island Volcanic Plateau

North Island Volcanic Plateau

The North Island Volcanic Plateau is a volcanic plateau covering much of central North Island of New Zealand with volcanoes, lava plateaus, and crater lakes....

 to the east and the valleys and gorges of the Whanganui River

Whanganui River

The Whanganui River is a major river in the North Island of New Zealand.Known for many years as the Wanganui River, the river's name reverted to Whanganui in 1991, according with the wishes of local iwi. Part of the reason was also to avoid confusion with the Wanganui River in the South Island...

 to the west.

South of Taumarunui

Taumarunui

Taumarunui is a town in the King Country of the central North Island of New Zealand. It is on State Highway 4 and the North Island Main Trunk Railway....

 the terrain

Terrain

Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...

 is steep but not unmanageable, with the exception of the stretch between Raurimu and National Park

National Park, New Zealand

National Park is a small town on the central plateau of the North Island of New Zealand. Also known as National Park Village it is the highest urban township in New Zealand at 825 metres. As the name suggests, it borders the World Heritage Tongariro National Park, New Zealand's first national...

, 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

Gradient

In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....

 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.

Construction

The problem was solved in 1898 by R. W. Holmes, Public Works Department

New Zealand Ministry of Works

The New Zealand Ministry of Works, formerly the Department of Public Works and sometimes referred to as the Public Works Department or PWD, was founded in 1876 and disestablished and privatised in 1988...

 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 the Piopiotea. 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 forms an ascending spiral southwards, with two tunnels, a circle and three hairpin bends. From the north, trains pass Raurimu before going round a 180° bend to the left in a horseshoe curve, climbing above the track on which they have just travelled. Two sharp bends to the right follow, after which the line passes through two short tunnels. Trains then complete a full circle, crossing over the longer of the two tunnels through which they have just passed, before continuing towards Wellington. Two kilometres further on the line has two further sharp bends, to the right and then to the left.

After the second of these bends a train has risen 132 m and travelled 6.8 km from Raurimu - the straight-line distance is 2 km.

Legend has it that a train driver once emergency-braked his train in the night upon mistaking the light of his last wagon on a nearby part of the spiral as the rear of a different train directly ahead of him.

External links

• NZ Engineering Heritage North Island Main Trunk line page
• Raurimu Spiral (NZR Publicity pamphlet)
• The New Zealand Railways Magazine, Volume 11, Issue 6 (September 1, 1936) digitised at NZETC