When the first X was completed in 1908 at NZGR's Addington Workshops in Christchurch, it was the very first 4-8-2 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, 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 initially operated as the freight counterpart of the passenger A class, but they struggled to operate at speeds higher than 50 km/h (30 mph). They were built as de Glehncompound locomotives, but during the 1940s, most of the class were converted to simple, superheated locomotives. This increased their power but did not prolong their lives and most were officially withdrawn from service on 2 March 1957, though a few had been taken out of service earlier and two were sold to the Ohai Railway Board (ORB) that operated a private industrial line at the end of the Wairio Branch.
The ORB locomotive, X 442, survived and has passed into the hands of railway preservation enthusiasts. It is currently located at the depot of the Feilding and District Steam Rail Society. Two X class boilers are held by Mainline Steam.