South Island Main Trunk

The Main North Line between Picton and Christchurch and the Main South Line between Lyttelton and Invercargill, running down the east coast of the South Island of New Zealand, are sometimes together referred to as the South Island Main Trunk Railway (SIMT). Construction of a main line running the length of the east coast began in the 1860s, not completed all the way from Picton to Invercargill until 1945.

Construction of the Main South Line began in 1865 when the Canterbury Provincial Railways began work on a 1,600 mm (5 ft 3 in) broad gauge line south from Christchurch. It reached Rolleston on 13 October 1866 and Selwyn a year later. A number of routes south were considered, and the one chosen was a compromise between a proposal to build a coastal line through fertile territory and a proposal to build an inland line to achieve easier crossings of rivers such as the Rakaia. However, construction had to be postponed as the Canterbury Province government was low on funds, and it did not restart until Julius Vogel announced the central government's "Great Public Works Policy".

The "Great Public Works Policy" placed a high priority on the completion of the Main South Line. At this time, New Zealand accepted 3 ft 6 in (1,067 mm) narrow gauge as its national rail gauge, but Canterbury was permitted to extent its broad gauge as far as Rakaia - although it did so on 2 June 1873, it converted its entire network to narrow gauge by 6 March 1876. Further south, the Dunedin and Port Chalmers Railway was opened on 1 January 1873 as the first railway in the country to adhere to the new national gauge. Although the final portion of this line became the Port Chalmers Branch, most of it was incorporated into the main line northwards and construction progressed through difficult terrain towards Oamaru. South of Dunedin, work was progressing on a link with Invercargill; a line between Invercargill and Gore was opened on 30 August 1875 and a line between Dunedin and Balclutha was opened two days later. Construction to link these sections faced more construction challenges than the earlier work had, and accordingly, the rate of progress slowed.

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