The class S1 was the most powerful steam shunter built for the South African Railways. It was designed by Dr. M.M. Loubser in World War II with the goal to produce an enforcement to the class S in the SAR's own Salt River workshops at Cape Town. Its boiler was larger than that of the class S and basically a shortened variant of the boiler that Loubser had fitted to the rebuilt class 12AR 4-8-2. Some parts, like the bar frames, tyres, superheater elements and smaller fittings still had to be imported. When completed, these locomotives were the most powerful shunting locomotives in the world which had a gauge smaller than standard gauge. They were able to move trains of up to 2,000 tons.

Due to the war, the first twelve locomotives were only built in 1947. Another order for 25 was placed with North British, these locomotives were completed in 1953 with the works numbers 27261 to 27285. Both batches can be distinguished by their running numbers, since the locomotives built in South Africa were numbered 374 to 385, while the Scottish locomotives were numbered 3801 to 3825. Originally they were spread over multiple depots, but from 1982 they could only be found at Germiston and its sub-depot at Kaserne. In the following years, all were withdrawn and only number 375 “Voortrekker” was preserved by Transnet Heritage Foundation. It is stored at Bloemfontain.