The Midland line is a 212 km section of railway between Rolleston and Greymouth in the South Island of New Zealand.

Railway development in the South Island in the 1870s was concentrated on a main line linking the established centres of Christchurch, Timaru, Dunedin and Invercargill and light, easily constructed branch lines serving the arable plains; (see Vogel Era).

The construction of the line was rejected in 1883 by a Royal Commission, who argued that despite the significant timber and coal resources of the West Coast of the South Island, the line would not be economic, in the face of limited resources during the Premiership of Sir Julius Vogel, although Parliament did pass the East and West Coast and Nelson Railway Act to enable private interests to construct a line.

Blair as "very rough, the mountain slope rises from the riverbed while the river runs in a fearful gorge all the way".

Cass was not reached until 1910 and Arthur's Pass township in 1914 – the Westland section meantime having advanced to Otira – to meet the other major obstacle.

Undeterred the government continued with construction, despite spiralling costs and labour shortages due to World War I.

The class were made famous by a documentary named "KB country", a term that has entered into New Zealand railfan jargon.

To house the larger and more numerous locomotives, Elmer Lane shed opened at Greymouth in 1928.

Increasing volumes of coal traffic led to the introduction of the DX class, a number specially modified for use in the Otira Tunnel.