South African class 21

In the late thirties A.G. Watson designed a heavy mixed-traffic locomotive that had to be limited to an axle load of 15 tons. As he had an aversion against articulated locomotives, it had to have five driving axles and a two-axle trailing truck to carry the weight of the firebox. This resulted in a 2-10-4 wheel arrangement and required some measures to allow it to run through the tight curves on branch lines. So the first driving axle had 1.75 inches of side play and the third and fourth ones had flangeless wheels. The boiler was the same Standard No. 3B design as the one used on the main line classes 15E, 15F and 23. To stay within the axle load limit, weight was saved by using a welded cab, by making the footplates and running boards of duralumin and by machining hollow axles and crank pins.

North British built only one in 1937 that was designated class 21 and numbered 2551. It was to get rotary cam poppet valve gear like other Watson designs, but W.A.J. Day who was now in charge changed that to Walschaerts valve gear. After some years it was fitted with large smoke deflectors. The locomotive itself had good running characteristics, what did not completely apply to the stoker-equipped FT tender. This had four central axles mounted in the frames and one radially adjustable axle on each end. It had a capacity of 5,587 imperial gallons of water and ten tons of coal. The heavier class 22 of the same wheel arrangement was never built.

No. 2551 was mainly used for the Rhodesian mail trains where it could use its power and fairly high speed. Unfortunately, it did not take long until the class GM 4-8-2+2-8-4 Garratts arrived and delivered better performances in this task. Then it came to the line between Pretoria and Eastern Transvaal, but there it turned out that eight-coupled locomotives were better suited for winding lines. In World War II, it got the special task of hauling heavy military trains. The locomotive was scrapped in 1952 after only 15 years of service. As the boiler was of a standard design, it was used as a spare for another locomotive.