When the trains became heavier and heavier, the NZR developed a powerful shunting locomotive as a tender locomotive. In order to keep the axle load low and to enable sufficiently large driving wheels for occasional use in front of passenger trains, the class W^F 2-6-4T tank locomotive was used as the basis and the second trailing axle was removed. After the class C had become too light for the ever heavier trains, it was also used for suburban trains. Half of the locomotives were retired between 1964 and 1968.

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 J^B 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 J^A 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.

J^A 1271 in September 2012 while crossing the Otaki River

Karora

The J^A 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.

Drawing with the original boiler (even if the year is wrong)

Railway and Locomotive Engineering, August 1897

After some British locomotives, the K class of 1877 was the first US locomotive to see service in New Zealand. Externally, compared to the British locomotives, it had many special features that caused a stir. These included the graceful bar frame, the swiveling trailing axle, many attachments located outside of the boiler and the rich decoration. The agile running characteristics in particular were apparently an advantage over the predecessors on the South Island of New Zealand. With the introduction of more powerful locomotives, they were used more on branch lines and some were moved to the North Island. After receiving new boilers, some with Belpaire fireboxes, they were retired in the 1920s.

Since the three Class G Garratts procured in 1928 could not convince due to various problems, a new, powerful locomotive for heavy passenger and freight trains was needed for the mountainous North Island Main Trunk Railway. In order to be able to avoid further problems with articulated locomotives, a heavy locomotive with a 4-8-2 wheel arrangement was initially planned. A major problem with the limited loading gauge of the Cape gauge routes was getting the most powerful locomotive possible to operate under these conditions. The locomotives were designated class K after the previous class of the same designation, supplied from the USA in 1877, was phased out towards the end of the 1920's.

The design that finally came to fruition had a boiler of the maximum practicable diameter. The trailing axle was replaced with a bogie so that a large firebox could also be carried. In order to be able to arrange the largest possible cylinders within the loading gauge, they were designed with a diameter of just 20 inches and a long stroke of 26 inches. The Heusinger valve gear initially caused problems due to the excessively long piston travel, but these could be remedied by reducing the travel from eight to 7.25 inches.

The total number of 30 engines replaced mainly the 4-8-2 Class X locomotives. They performed very well with heavy trains and were mostly converted to oil firing after the Second World War due to the lack of coal. The changes made were carried out in parallel with their successors of the K^A class. Since there were frequent frame fractures in the area of the firebox in the K class, new frames of the same design as in the K^A were installed.

Although the switch to diesel traction took place in the mid-fifties, several examples of the class were still being overhauled at that time. All engines were finally decommissioned between 1964 and 1967. A total of three examples were preserved, two of which were last used as stationary steam generators. One of these two engines, as well as one that was recently exhibited in a museum, were selected for a renewed refurbishment in 2013 and 2014 respectively.