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 collectively as the South Island Main Trunk Railway (SIMT). Construction of a line running the length of the east coast began in the 1860s and was completed all the way from Picton to Invercargill in 1945; the last sections being on the Main North Line south of Picton. The designation "South Island Main Trunk" originally referred to only that line between Christchurch and Invercargill. Construction Main South Line Construction of the Main South Line (originally known as the "Great Southern Railway") began in 1865 when the Canterbury Provincial Railways began work on a 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 the fertile territory and a proposal to build an inland line to achieve easier crossings of rivers such as the Rakaia. 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, the central government had decided on narrow gauge as New Zealand's national rail gauge, but Canterbury was permitted to extend 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 mainline 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.