The Marton New Plymouth Line was originally part of the main line section from Wellington to New Plymouth, the first part of which was constructed and operated privately as the Wellington and Manawatu Railway from the 1880s until 1908.
Construction of the route north-west of Longburn as part of the NZR network commenced in the 1870s with the first section to Feilding opened in 1876. Feilding-Marton was opened in 1878. The next section was built south from Whanganui, opening to Okoia in 1877 and Marton in 1878. Heading north, Whanganui-Kai Iwi opened 1879, Kai Iwi-Waitotara opened 1880, Waitotara-Waverley opened 1881, Waverley-Manutahi opened 1883.
The rest of the route was constructed south from Waitara: Waitara-Lepperton-New Plymouth opened 1875, Lepperton-Inglewood opened 1877, Inglewood-Stratford opened 1879, Stratford-Ngaere opened 1880, Ngaere-Eltham, Eltham-Normanby and Normanby-Hawera opened 1881. By 1883 when Manutahi was reached from the south, there still remained a difficult 16 km section incorporating the Manawapou and Tangahoe viaducts and it was not until 1885 that the Manutahi and Hawera railheads were joined into one continuous line from Wellington right to New Plymouth. In 1933 the route became part of a loop around the NIMT when the Stratford Okahukura Line opened from Stratford near New Plymouth, to Okahukura just north of Taumarunui. It was used as a bypass route for NIMT trains on numerous occasions over 76 years. However the SOL section was closed to trains in 2009.
Once the complete Marton to New Plymouth section was open, its operational limitations became clear, and by 1894, improvements were being considered. An engineers’ report published that year in the Wanganui Chronicle of 5 September focused specifically on Turakina-Waitotara where four different deviations were recommended. The first of these became the Turakina-Okoia deviation but was not commenced until 1937 and completed 1947. The Westmere Bank, which is a steep climb from Kai Iwi (north side) and Aramoho (south side) to Westmere (formerly Brunswick), with maximum gradient of 1 in 28, was proposed to be addressed by a new section from Aramoho to Brunswick (Westmere) which as far as we know was never built. Also unimplemented was a proposed deviation from Kai Iwi to Okehu (Maxwell) which was to have incorporated a tunnel about 1.6 km long, and a realignment from Nukumaru to Waitotara.
However in 1911 a deviation of about 3.6 km was started at the Manawapou Viaduct when that structure was replaced on a new alignment and altitude (over 8 metres higher), easing gradients and curvatures. It was completed about May 1914. 50 metres of it was washed away by flooding in March 1966, and restored with temporary spans until permanent reconstruction in 1969. The nearby Tangahoe viaduct in wood is thought to have been replaced in the early 1910s (yet to be confirmed). The Whenuakarua bridge between Patea and Waverley, originally of wooden construction, was partly washed out by a flood in 1922. It was temporarily reinstated, and then completely replaced in 1930.
A smaller deviation was undertaken at the Kai Iwi tunnel (no.4) with work beginning there in 2007 over a distance of 1 km. The work was required to increase rolling stock clearances and train speeds. A new alignment approximately 150 metres west of the tunnel included a cutting 300 metres long and 30 metres wide. It was opened to traffic in 2008. The tunnel remains abandoned at its original site.
Marton became a three way junction in 1888 with the opening of the southernmost section of the NIMT to Kaikarangi. In later years the Marton New Plymouth Line origination was shifted from Wellington to Marton as the line south of Marton was reclassified part of the North Island Main Trunk from Wellington to Auckland. However, mile peg and bridge/tunnel number sequences did not reflect this change until metrication in 1974.