Mangaweka deviation

The Mangaweka Deviation is a 7 km single track deviation of the North Island Main Trunk (NIMT) railway line in the central North Island of New Zealand, between the settlements of Mangaweka and Utiku, south of Taihape. Opened on 18 November 1981, it was constructed between 1973 and 1981; to move the line away from geologically unstable land; and also to replace the high-maintenance steel viaducts including the Mangaweka Viaduct. It is the most recent and most significant deviation of the NIMT since it was opened in 1909.

The original section of the line was constructed as part of the NIMT in 1904. The entirety of the route from Marton north to Taihape was built to the west of the Rangitikei River and lies predominantly on the terraces within the river valley. Only south of Marton does the NIMT cross the Rangitikei en route to Palmerston North and Wellington.

Between the settlements of Mangaweka and Utiku the western river terrace is largely non-existent with steep hill country running into tall river cliffs. The topography is also compounded by the confluence of the Kawhatau River with the Rangitikei. However, despite these obstacles surveyors were able to identify a route that kept the line to the west of the river along the cliff tops; albeit one that included a climb at each end of the section, a viaduct, and several tunnels.

By the 1960s concerns existed about the long-term stability of the land the section ran through. While the general Mangaweka area was susceptible to slips due to its steep hilly nature, rainfall and underlying geology of papa rock (mudstone), the railway, by virtue of its cliff top nature and several tunnels, was thought to be under particular threat from erosion and collapse.

At the same time concerns were also being voiced over State Highway 1 as the steep, narrow and twisting alignment was also prone to slips – having been built after the railway in the same area though further to the west – and naturally imposed speed restrictions that were exacerbated by heavier vehicles and creating bottlenecks.

...
Wikipedia