The class B was a 4-8-0 freight locomotive designed for the main lines of the South Island. Starting in 1899, four were delivered by Sharp, Stewart & Co. and six more by the NZR Addington shops. They were capable of hauling 600 tons on fairly flat terrain. As early as 1903, three were rebuilt to the 4-6-4T class WE. Starting in the twenties, some got superheated boilers. Some of these even got more powerful superheated boilers with a wide firebox and a pressure raised from 170 to 200 psi.
Between 1911 and 1913, the Addington shops delivered ten locomotives of the class BA. These had a superheater and a smaller firebox. Their haulage capacity was now 700 tons. They were now increasingly used on branch lines and often also hauled passenger trains. Like some class B locomotives, they also got wide firebox boilers starting in 1928.
The largest class of the series was the BB that was built by A & G Price of New Zealand. 30 were completed between 1915 and 1918. While the others had been used on the South Island, the BB was mainly intended for freight and passenger trains on branch lines on the North Island. Their cylinder diameter had been increased by one inch and the firebox hand been enlarged on the cost of a smaller tube heating area. Without the introduction of the AB class Pacifics, it is possible that more would have been ordered.
In the thirties, eight BA and ten BB were modified for use as shunters. The withdrawals of all three classes took place in the sixties and had been completed in 1969. No. 552 is the only member of class BA that has been preserved. It is being owned by the Mainline Steam Heritage Trust and has been operational, but currently is being overhauled. Also BB No. 144 is currently at the same trust's depot at Parnell and is awaiting its restoration.