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, 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 Taumarunui to Taihape section for some years, as the track north and south was rather than and their trains were restricted to for passengers and for freight. This caused frustration to general manager Hiley who wanted them used over the entire NIMT (they had had to be partially dismantled for their initial journey to Taihape). The X class initially operated as the freight counterpart of the passenger
A class, but they struggled to reach speeds higher than . They were built as
de Glehn compound locomotives, but during the 1940s most of the class were converted to simple-expansion
superheated locomotives. This increased their power but did not prolong their lives and most were officially withdrawn on 2 March 1957. However, a few had withdrawn before this and two were sold in 1946 to the
Ohai Railway Board (ORB), which operated a private industrial line at the end of the
Wairio Branch. == Preservation ==