The class X was the first type of 4-8-2 tender locomotives newly built anywhere in the world. A.M. Beattie designed it for heavy freight traffic on the mountainous central section of the North Island Main Trunk Railway. 18 were built at Addington Workshops between 1908 and 1909 and between 1913 and 1915. This is why some say this class was the origin of the name “Mountain” for this wheel arrangement, but others attribute this name to the Chesapeake & Ohio which introduced their class J-1 in 1911.

Unlike later 4-8-2 locomotives, the class X still had its firebox above the last driving axle and not behind it. They were De Glehn compounds with two high pressure cylinders with a diameter of 13.5 inches on the inside below the smokebox which drove the first driving axle. The outside low pressure cylinders with a diameter of 22 inches were a bit further back and drove the second driving axle. The drivers had a diameter of only 45 inches.

For some years, these locomotives were constrained to a 93-mile section since the tracks on both ends of the mountainous section were too light for them, until these were upgraded. They usually hauled freight trains, while the class A Pacifics hauled passenger trains. The class X sometimes also hauled passenger trains, but rarely exceeded 30 mph or 48 km/h. There are reports that they did good work, but were not liked by firemen due to the shape of their grate. 

Between 1943 and 1949, eleven were superheated and rebuilt to four-cylinder simples. For this they received outside cylinders which also had a diameter of 13.5 inches. Some received thermic syphons in this process. In 1946, two of the rebuilt locomotives were sold to the Ohai Railway Board. The NZR withdrew the last ones in 1957, while the Ohai Railway Board did the same in 1968. X 442 was sold into preservation, but is not operational today.