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).^[1] 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.

The South Island Main Trunk has been famous for its passenger services. In the days of steam locomotives, the South Island Limited expresses were particularly famous; drivers of J and J^A class locomotives claimed to have broken the official New Zealand railway speed record on a section of track near Rakaia called the "racetrack". The Main South Line saw the last regularly steam-hauled expresses in New Zealand, with J^A locomotives hauling the Friday and Sunday night expresses until 26 October 1971. All other steam-hauled expresses were replaced on 1 December 1970 by the Southerner, which was hauled by DJ class diesel-electric locomotives. This service was one of the most famous in New Zealand, but it ceased on 10 February 2002.

For many years, Fiat or 88-seater railcars ran services on the Main North Line, but they were withdrawn during the 1970s. On 25 September 1988 the tourist-focused Coastal Pacific express began operating along the scenic route between Christchurch and Picton; it continues to operate to this day. In the summer of 1994–95, this service was augmented by the Lynx Express, which was unsuccessful and not repeated in later years. Commuter services used to operate around major centres along the South Island Main Trunk, and many rural services also operated when country branch lines were operational, but the branch lines progressively closed during the 20th century and commuter services in the South Island ceased in the 1980s. Nowadays, the only long-distance passenger services on the South Island Main Trunk are the Coastal Pacific and the TranzAlpine, which uses the short portion of the Main South Line between Christchurch and Rolleston before running along the Midland Line to Greymouth.

Freight services on both lines operated for many years as feeder services from rural districts to nearby major centres and harbours, rather than utilising long-distance services between the important cities. The first through freight from Christchurch to Invercargill did not operate until December 1970. During the 1970s and 1980s, patterns of freight haulage changed dramatically, with the last of the branch lines closing and an emphasis placed upon long-distance haulage.^[5] The South Island Main Trunk is now used to carry significant quantities of long-distance freight, and it connects with the North Island via roll-on roll-off ferries between Picton and Wellington. These ferries have allowed freight trains to be operated from Auckland to Christchurch on a 30-hour schedule.