In 1908, a heavy and powerful locomotive was required to haul traffic on the newly completed mountainous central section of the North Island Main Trunk Railway, and as a logical progression of the 4-6-2 Q class design, the 4-8-2 wheel arrangement was created for the X class.

[1] When the first X was completed in 1908 at NZR's Addington Workshops in Christchurch, it was the very first 4-8-2 tender locomotive built in the world.

The 4-8-2 design went on to be popular in the United States and was nicknamed the "Mountain" type; one theory suggests this name stems from the mountainous terrain that inspired the X's design,[1] while another suggests the Chesapeake and Ohio Railway first coined the name in reference to its 4-8-2s of 1911 that were built to operate in the Allegheny Mountains.

The X class were restricted to the 93 miles (150 km) Taumarunui to Taihape section for some years, as the track north and south was 53 lb/yd (26.3 kg/m) rather than 70 lb/yd (34.8 kg/m); and their trains were restricted to 25 mph (40 km/h) for passenger trains or 20 mph (32 km/h) for goods trains; a source of frustration to general manager Hiley who would have liked to run them over the entire NIMT (they had had to be partially dismantled for their initial journey to Taihape).

[3] This increased their power but did not prolong their lives and most were officially withdrawn from service on 2 March 1957,[3] though a few had been taken out of service earlier and two were sold to the Ohai Railway Board (ORB) in 1946 that operated a private industrial line at the end of the Wairio Branch.