The North Island Main Trunk (NIMT) is the main railway line in the North Island of New Zealand, connecting the capital city Wellington with the country's largest city, Auckland. The line is long, built to the New Zealand rail gauge of and serves the large cities of Palmerston North and Hamilton.

Around approximately 75%of the line is electrified in three separate sections: one section at 1500 V DC between Wellington and Waikanae (55 km, 34 mi), and two sections at 25 kV AC: between Palmerston North and Te Rapa (Hamilton) and between Pukekohe and Auckland Britomart.

The first section of what became the NIMT opened in 1873 in Auckland. Construction at the Wellington end began in 1885. The line was completed in 1908 and was fully operational by 1909. It is credited for having been an economic lifeline, and for having opened up the centre of the North Island to European settlement and investment. In the early days, a passenger journey between Wellington and Auckland could take more than 20 hours; today, it takes approximately 11 hours.

The NIMT has been described as an "engineering miracle", with numerous engineering feats such as viaducts, tunnels and a spiral built to overcome large elevation differences with grades suitable for steam engines, the ruling gradient being 1 in 50. When built the Makatote Viaduct was the highest viaduct in the country.

History

Construction

When the first sections of the NIMT were built, there was great uncertainty as to whether the initial terminus on the Waikato River would be Mercer, or Tuakau (see 1871 map of possible routes), and, within Waikato in 1872, Cambridge, Kihikihi, Te Awamutu and Alexandra were possible destinations. The central section was gradually extended to meet up in 1909, 23 years after the last of the northern and southern sections of NIMT had been opened.

Auckland to Te Awamutu Auckland's first railway was the line between Point Britomart and Onehunga via Penrose, opened in 1873. It was built by Brogdens, as was the rest of the Auckland & Mercer Railway, for £166,000 for the to Mercer. The section from Penrose to Onehunga is now called the Onehunga Branch. The line was later continued south from Penrose into the Waikato. To support the Invasion of the Waikato, a tramway was built from Maungatāwhiri to Meremere in 1864, with a first sod event near Koheroa on Tuesday, 29 March 1864 by Auckland's Chief Superintendent of Roads & Bridges, W R Collett. Turning of the first sod of the Auckland and Drury Railway took place in 1865, a year after the last major battle. By 1867 the Provincial Government had run out of money and an Amendment Act was passed. In 1870, the resident engineer proposed cutting costs by reducing the gauge from standard to 3 ft 6 in gauge, though rolling stock had been bought for the wider gauge. In 1872 contracts were let to Owen Jones and to Brogdens and work was soon reported as well under way. This line reached Mercer by 20 May 1875, with from Ngāruawāhia being constructed by the Volunteer Engineer Militia and opened on 13 August 1877. It was extended to Frankton by December 1877, to Ōhaupō on 4 June 1878 and to Te Awamutu on 1 July 1880. An economic downturn stalled construction for the next five years, and Te Awamutu remained the railhead. There were negotiations with local Māori, and the King Country south of Te Awamutu (from the boundary of the Puniu River) was not accessible to Europeans until 1883.

Wellington to Marton The Longburn to Marton section had been opened on 18 April 1878, as part of the line linking the ports of Foxton and Whanganui.

The Wellington-Longburn (near Palmerston North) section was constructed between 1881 and 1886 by the Wellington and Manawatu Railway Company (WMR). The company was acquired by the government and merged with the New Zealand Railways Department in 1908.

Central North Island

In 1882, the Whitaker Ministry intiated the North Island Main Trunk Railway Loan Act, which was passed by the General Assembly, to expedite construction of the North Island Main Trunk south of Te Awamutu by authorising the overseas borrowing of a million pounds for the work, with the final route to be approved by the General Assembly following surveys of potential routes, for which £10,000 were allocated.

From Te Awamutu, it was proposed that the line be built via Taupo or via Taumarunui, the eventual route. Four options were considered before the Minister of Public Works decided on the present route in 1884, but, when it was realised just how difficult that route was, further surveys considered two other options in 1888. Routes via Napier and Waitara were also considered, with surveying in Taranaki from 1883 to 1889.

Construction of the final central section began on 15 April 1885, when paramount chief Wahanui of Ngāti Maniapoto turned the first sod outside Te Awamutu. It was 23 years before the two lines met, as the central section was difficult to survey and construct. The crossing of the North Island Volcanic Plateau with deep ravines required nine viaducts and the world-famous Raurimu Spiral.

Richard Seddon’s Liberal Government pledged in 1903 that the whole route would be open in 1908. In 1904, the railheads were still apart, and contracts for three massive viaducts (Makatote, Hapuawhenua and Taonui) were not let until 1905. The government committed 2500 workmen, and in 1907, the Minister of Public Works William Hall-Jones instigated a night shift (under kerosene lamps). By the beginning of 1908, there was a gap between Erua and Ohakune, with a connecting horse-drawn coach service. From Ohakune south to Waiouru, the Public Works Department operated the train, as this section of had not yet been handed over to the Railways Department.

Opening The gap was closed on 7 August 1908 for the first through passenger train, the 11-car Parliamentary Special carrying the prime minister Sir Joseph Ward and other parliamentarians north to see the American Great White Fleet at Auckland. But much of the new section was temporary, with some cuttings north of Taonui having vertical batters and unballasted track from Horopito to Makatote. Ward drove the last spike on 6 November 1908, and the 'Last Spike' monument is at Manganui-o-te-Ao , near Pokaka. A two-day NIMT service started on 9 November, with an overnight stop at Ohakune (for northbound passengers) or Taumarunui (for southbound passengers).

On 14 February 1909, the first NIMT express left Auckland for Wellington, an overnight trip scheduled to take 19 hours 15 minutes, with a sleeping car, day cars with reclining seats, and postal/parcels vans. The dining car went on the north express from Wellington to Ohakune, then transferred to the southbound express, so avoiding the heavy gradients of the central section.

Upgrades and deviations Signals and track Most of the NIMT is single track with frequent passing loops, but sections at each end that also handle suburban commuter traffic are double tracked:

The section known as the North-South Junction between Wellington and Waikanae, except for of single-track through tunnels between North Junction ( from Wellington) and South Junction, ( from Wellington), on the Pukerua Bay to Paekākāriki section, between Hamilton and Te Kauwhata (except for the single-track Waikato River Bridge at Ngāruawāhia), and between Meremere and Maungawhau.

The rails and signalling have been upgraded over the years, and many sections of the line have been deviated: The original 1870s Vogel Era track had rails of 40 lb/yd (19.9 kg/m), some were iron not steel; later rails were 53 lb/yd (26.3 kg/m); and from 1901 70 lb/yd (34.8 kg/m), e.g. between Taumarunui and Taihape for the heavy X class locomotives used on the central mountainous section from 1908. Some 10 bridges between Frankton and Taumarunui had to be strengthened, and in 1914 there was still 129 km (80 mi) of 53 lb/yd rail to be replaced. In the 1930s 85 lb/yd (42.2 kg/m) was adopted, then 91 lb/yd (45.1 kg/m), and from 1974 100 lb/yd (50 kg/m).

Signalling on the single-track sections (most of the line) was controlled by Tyer's Electric Train Tablet No 7 system; with each of the stations for the 94 tablet sections staffed by three tablet porters each working a 56-hour week for continuous coverage; hence each station required at least four houses for the stationmaster and three porters. Pierre noticed that with CTC station buildings and even platforms had been removed as there were no longer any staffed stations between Ohakune and National Park. The Train Control system introduced from 1928 to 1932 supplemented the tablet system by operators at the four sections (Auckland, Frankton, Te Kuiti, Ohakune, Marton and Wellington) to expedite operation of trains over several tablet sections; the 1925 Fay-Raven report urged its adoption because of the fitful progress of mixed trains, with locomotives often kept waiting. From 1938 to 1966 Centralised Traffic Control (CTC) gradually replaced the tablet system on the NIMT. In 1957 when the installation of CTC over the remaining commenced, it was estimated that using CTC over the Taumarunui to Otaki section with control centres at Ohakune (which shifted to Taumarunui in 1977), Taihape and Palmerston North would replace 74 men in traffic working duties. The last section converted was Piriaka-Owhango.

A 1926 article by "Backblocks" described conditions for staff at these stations where four workers lived in isolated areas, and their efforts to get special trains for transport to special events.

In 1913, the maximum speed limit on the NIMT was raised to , reducing the journey time by 1 hour 25 minutes Auckland-Wellington or to 17 hours and between 30 and 45 minutes. Under Thomas Ronayne, the New Zealand Railways Department general manager from 1895 to 1913, the section south to Parnell was duplicated and improvements made to the worst gradients and tight curves between Auckland and Mercer. Under his successor E. H. Hiley the second Parnell Tunnel with two tracks and an easier gradient was completed in 1915–1916. On the Kakariki bank between Halcombe and Marton a deviation reduced the 1 in 53 grade to 1 in 70 in 1915. Similar work was done to ease the gradient to Greatford, on the other side of the Rangitīkei River, in 1939. A 1914 Act authorised spending on the Westfield Deviation, new stations at Auckland and Wellington, track doubling (Penrose-Papakura, Ohinewai-Huntly, Horotiu-Frankton, Newmarket-New Lynn), and grade easements from Penrose to Te Kuiti, but the war delayed most of these works for over a decade.

In 1927, automatic colour-light signalling was installed from Otahuhu to Mercer. In 1930 the signalling was extended to Frankton and the from there to Horotiu was doubled. The north to Ngāruawāhia was doubled from 5 December 1937, followed by Ngāruawāhia to Huntly on 4 December 1938 and Huntly to Ohinewai and Papakura to Paerata in December 1939. By then, wartime shortages delayed further double-tracking. Pokeno to Mercer was doubled from 11 November 1951, Pukekohe to Pokeno 21 November 1954, Mercer to Amokura 1 July 1956 and Ohinewai to Te Kauwhata 14 December 1958. The between Amokura and Te Kauwhata remain single track, as does Ngāruawāhia bridge. Doubling of the section south of Amokura is being investigated in a business case from July 2021.

Westfield deviation

In 1930, the Westfield deviation was opened, creating a new eastern route from Auckland to Westfield via Glen Innes and Hobsons Bay, running into the new Auckland railway station and providing better access to the Port of Auckland. The original section between Auckland and Westfield via Newmarket later ceased to be part of the NIMT: Auckland to Newmarket became the Auckland-Newmarket Line, and Newmarket to Westfield became part of the North Auckland Line (NAL) which runs between Whangarei and Westfield.

In the late 1930s, bridges replaced level crossings at Ohinewai, Taupiri and Hopuhopu.

Tawa flat deviation