The Main North Line, sometimes referred to as part of the South Island Main Trunk railway, is a railway line that runs north from Christchurch in New Zealand up the east coast of the South Island through Kaikōura and Blenheim to Picton. It is a major link in New Zealand's national rail network and offers a connection with roll-on roll-off ferries from Picton to Wellington. It was also the longest railway construction project in New Zealand's history, with the first stages built in the 1870s and not completed until 1945.

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.