The NZR class J of 1939 was designed to pull both express and freight trains on branch lines, but also be fast enough for use on main lines. They had a 4-8-2 wheel arrangement and were built with partial streamlining, which was later removed. The JB class was created by converting twelve class J locomotives to burn oil.
After 40 class J locomotives that were built at North British in 1939, 51 of the improved class JA were built between 1946 and 1956. These had no streamlining and were partly built in the NZR Hillside works. The 16 locomotives built in Scotland had oil firing. The intended area of operation now only included express trains.
The JA was the last class of steam locomotives that the NZR purchased. Diesel railcars and locomotives had already being introduced at the time the last ones were delivered. Although they were too large for some branch lines, they could also hold their own against heavier locomotives on main lines. Although the speed limit for express trains in New Zealand was 55 mph (89 km/h), they are said to have regularly reached well over 70 mph (113 km/h). Many survived until the end of the steam age on the NZR in 1971. Today ten remain, several of which are operational.
