The 4-8-2 locomotive of the class 12A was the last design by D.A. Hendrie for the South African Railways. It was based on the class 12 that had been introduced before World War I. The biggest change was that the area of the firebox was enlarged by 30 percent. All in all, they had the maximum size that was possible for a Mountain locomotive on cape gauge. 

Between 1919 and 1929, North British built 48 and Henschel built 19 more. At the beginning, they were used for coal trains on the line between Witbank and Germiston. Later they were also used for passenger trains and spread over all South Africa. Starting in 1943, a total of 44 were rebuilt to class 12AR. Since there was no Watson standard boiler in a suitable size, they got a completely new boiler.

All in all, both the unrebuilt class 12A and the rebuilt class 12AR were reported to be very free steaming and powerful for a cape gauge Mountain. Between 1947 and 1953, North British built eight similar locomotives without superheater for industrial operators. In SAR service, the remaining class 12A and 12AR locomotives had all been moved to shunting service by 1977. The last ones were withdrawn in the eighties.