The line is 682 kilometres (424 mi) long, built to the New Zealand rail gauge of 1,067 mm (3 ft 6 in) and serves the large cities of Palmerston North and Hamilton.

To support the Invasion of the Waikato, a 3.5 mi (5.6 km) tramway was built from Maungatāwhiri to Meremere in 1864,[10] with a first sod event near Koheroa on Tuesday, 29 March 1864 by Auckland's Chief Superintendent of Roads & Bridges, W R Collett.

[17] 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.

The government committed 2500 workmen, and in 1907, the Minister of Public Works William Hall-Jones instigated a night shift (under kerosene lamps).

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.

[26] Under Thomas Ronayne, the New Zealand Railways Department general manager from 1895 to 1913,[27] the section south to Parnell was duplicated and improvements made to the worst gradients and tight curves between Auckland and Mercer.

[30] 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,[31] but the war delayed most of these works for over a decade.

With a pair of tunnels under the Wellington hills, the deviation alleviated issues with more and heavier freight traffic on the steep and twisting original route where long sections at 1 in 60 gradient required banker engines.

A Centralised Train Control (CTC) system was installed in 1940, so that new signal boxes were not required and five stations between Tawa and Pukerua Bay no longer had to be continually staffed for Tablet operation; see Kapiti Line and North–South Junction.

The difficult section down the Paekakariki Escarpment from Pukerua Bay to Paekākāriki with five tunnels between South and North Junctions remains single track.

[41] In 1967, the floors of the tunnels on the former WMR section between Paekākāriki and Pukerua Bay were lowered to enable the DA class locomotives to travel all the way to Wellington.

[45] The central section from Te Rapa near Hamilton to Palmerston North was electrified between 1984 and 1988 as part of the Think Big government energy program.

In February 2011, duplication between Paekākāriki and Waikanae was completed as part of the upgrade and expansion of the Wellington suburban network; see Kapiti Line for more information.

Electrification of the NIMT was mooted by electrical engineer Evan Parry in the first volume of the New Zealand Journal of Science and Technology in November 1918.

In light of a national coal shortage following World War I, Parry argued that the network was under great strain due to ever-increasing volumes of freight, and the use of steam traction was partly to blame.

Electric traction in this section is now used only by Transdev Wellington for Metlink suburban passenger services on the Kapiti Line, and was extended to Paraparaumu on 7 May 1983 and Waikanae on 20 February 2011.

Funded by the Greater Wellington Regional Council, the extension to Waikanae coincided with the delivery of new FP class Matangi electric multiple units.

From 1948 to 1951 the General Manager of the Railways Department , Frank Aickin advocated electrification of the entire line, despite protests from his engineering staff.

Aickin had previously been Staff Superintendent and Chief Legal Advisor to the Department and considered using diesel locomotives for trains on the NIMT to be too expensive.

The 411 km (255 mi) section between Palmerston North and Hamilton was electrified at 25 kV 50 Hz AC, opened on 24 June 1988[52] as one of the Muldoon National Government's "Think Big" energy development projects.

The economics of the project was greatly undermined by the fall of the price of oil in the 1980s and the deregulation of land transport, which removed the long-distance monopoly NZR held when the cost-benefit report was written.

The electrification of the section, which had its genesis in a study group set up in June 1974 to report on measures to be taken to cope with increasing rail traffic volumes, received approval in 1980.

In 1994 New Zealand Rail Limited sold the cable to Clear Communications for telephone traffic, leasing part of it back for signalling.

[57] The electrification project on the Auckland network, including the Auckland-Papakura section of the NIMT, was completed in July 2015, with all suburban services being electric.

[58] In February 2008 former Auckland Regional Council Chairman Mike Lee suggested the initial electrification might be extended to Pukekohe, leaving a 60 kilometres (37 mi) gap to Te Rapa.

[67] The reasons given for the decision included the fact that the EFs are now close to their end of life (approximately 30 years old) and suffer from frequent breakdowns (on average every 30,000 kilometres (19,000 mi) which is well below the expected breakdown-free service interval of 50,000 kilometres (31,000 mi)) and that having to change from a diesel locomotive to an electric one and back again at each end of the electrified section is labour-intensive, time-consuming and adds to costs.

On 30 October 2018, the Government announced that it would retain the EF class electric locomotives, to help meet its long term emissions goals and boost the economy.

This included the building of the famous Raurimu Spiral to allow trains to ascend the steep grade from the Whanganui River valley to the North Island Volcanic Plateau.

In 1968, a Drewery NZR RM class articulated 88-seater railcar was refurbished and repainted in a distinctive blue-and-white scheme that led to it being nicknamed the Blue Streak.

In June 2012, KiwiRail announced the Overlander would be replaced by the Northern Explorer, with modern New Zealand-built AK class carriages to provide a premium tourist train on a quicker timetable with fewer stops.