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.
As a result, investigations were commenced into improving both routes, including the feasibility of a joint rail-road alignment.
Construction and new alignment
From a number of options put forward different routes were chosen for SH1 and the NIMT, as it proved possible to reconstruct SH1 over much of the existing alignment. The SH1 works were completed in stages between 1972 and 1980.
The NIMT route chosen was from one of the joint options given and was constructed between 1973 and 1981. The new alignment crossed the Rangitikei River twice and the Kawhatau River once - requiring the construction of three massive viaducts - and also removed the gradients at each end of the section.
From the south, the deviation leaves the old alignment at exit of the Mangaweka tunnel, and crosses SH1 and passes much closer to the settlement than it did previously. It then passes under SH1 before crossing the South Rangitikei viaduct, which is a 78m high, 315m long triple-span structure. The line then runs along the eastern river terrace for about 2 km, firstly through a lengthy cutting and then across the 'Blind Gully' embankment (New Zealand's largest) before crossing the Kawhatau (73m high) and North Rangitikei (81m high) viaducts in quick succession; the North Rangitikei viaduct is the highest on the NIMT. Both the Kawhatau and North Rangitikei viaducts have a 160m long single centre span and two 25m long approach spans, and are to a cantilever design so differ from the South Rangitikei Viaduct design. The line rejoins the original NIMT alignment a further 1 km north, just to the south of Utiku.
The section was subsequently electrified as part of the NIMT electrification carried out between 1984 and 1988.
Following the deviation’s opening in 1981 work commenced on the removal of the former alignment with the lifting of the line. The Mangaweka viaduct was demolished in 1982.
The former alignment today
Much of the former alignment still exists today. For most travellers the most visible remnant of the old line are the bases of the Mangaweka viaduct which sit on the hillsides on each side of SH1 just north of the railway overpass. The formation can still be followed either side of where the viaduct stood, though to the south it is slowly being reclaimed by bush and the tunnel at the top of the climb has been almost totally blocked by a slip at the northern portal. North of the former viaduct the formation runs through farmland. The five tunnels on this side of the alignment remain largely intact.
- Merrifield, Rob (2009). "A Centennial Review of the North Island Main Trunk Railway - Geology of the West-Central North Island and its Influence on Transport Development" (PDF). 3rd Australasian Engineering Heritage Conference.
- The Old Mangaweka Section in Exploring New Zealand's Ghost Railways by David Leitch and Brian Scott pp28–29 (1995; 1998 revised edition, Grantham) ISBN 1-86934-048-5
- North Island Main Trunk by Bill Pierre pp. 199–202 (Reed, Wellington, 1981) ISBN 0-589-01316-5
- Rails: The Great New Zealand Adventure by Roy Sinclair pp 38,148 (1987, Grantham House, Wellington) ISBN 1-86934-013-2