The NZR class A^B was a class of 141 superheated two-cylinder Pacifics. Its origins can be traced back to the class A of 1906 that was a saturated four-cylinder compound. No. 409 of this class was built as a simple two-cylinder and in 1915, production of the superheated class A^B started. By 1927, 38 had been built in the NZR Addington Workshops, 83 by North British and 20 by A & G Price. The locomotives from North British differed from the rest in a firebox area of 124 instead of 115 square feet and a superheater that was only 155 instead of 204 square feet.

A feature of these locomotives was the Vanderbilt tender. They were the largest class of NZR steam locomotives and were used in passenger and freight service. On relatively flat lines, they could handle 350-ton express trains at 60 mph or 97 km/h and were able to haul freight trains of 750 tons. Some claim that they were the first steam locomotives to reach a power of 1 hp for every 100 pounds of empty weight. As loco-info's power estimation leads to 0.0096 hp per pound of service weight, this claim might actually be true.

The superheated standard boiler of the A^B was later also fitted to locomotives of the classes Q and A^A. The class W^AB was a 4-6-4T tank locomotive that was built in parallel to the A^B with the same boiler. Between 1947 and 1957, eleven of these were rebuilt to the A^B.

The most time of their career, they were used on the North Island. When diesel locomotives arrived there around 1955, they came to the South Island. In the late fifties and early sixties, some were withdrawn and others with damaged frames got the frames from these withdrawn locomotives. They spent their last years on branch lines and between 1963 and 1967, the largest number was withdrawn. The last ones survived until the end of steam in 1971. Today seven are preserved, with three currently operational.