In 1904, the Natal Government Railways procured a total of 50 class B locomotives for the main line, which has a gradient of up to 1 in 30. These had the wheel arrangement 4-8-0 and delivered sufficient power for the required tasks. For faster passenger trains, however, the locomotives were not smooth running due to the overhang behind the last coupled axle. 

For this reason, the chief engineer D.A. Hendrie fitted six pieces of the class with an additional trailing axle, which was in a Bissel frame. This was the world's first tender locomotive with a 4-8-2 wheel arrangement. In contrast, the American 4-8-2 “Mountain” had evolved from the 4-6-2 “Pacific” around 1910, but too served the purpose of hauling heavy passenger trains on mountain routes.

With the formation of the South African Railways, the converted locos became class 1B while the originals were classified as class 1. After new locomotives took over the tasks of class 1B in the second half of the 1920s, the trailing axles were removed again and the locomotives were thus restored to their original condition. They were now used together with the other class 1 locomotives in shunting service and were only completely retired until 1975. One example, which was in use at a sugar mill until 1984, still exists today.