The most outstanding feature of the NIMT (North Island Main Trunk) is the Raurimu Spiral on the central section 25 miles south of Taumarunui. A testing ground for any locomotive, the spiral has one complete circle, three horseshoe curves, and two tunnels, all on an average grade of 1 in 52. Between Raurimu and National Park, southbound trains climb 700 feet in only 7 miles.

A total of 71 class K, Ka, and Kb 4-8-4, three and a half foot gauge locomotives were built between 1932 and 1950, most by the Hutt Workshops on the North Island, but six were built at the Hillside Workshops in 1939 for use on the South Island. Retirements and scrapping began in 1964, as the GM G12 diesels began arriving. Five 4-8-4 locomotives were saved from scrapping; the original K number 900, Ka's 935, 942, 945, and Kb 968.