[1][2][3] The Class 21 2-10-4 Texas type locomotive was designed by A.G. Watson, Chief Mechanical Engineer (CME) of the South African Railways (SAR) from 1929 to 1936.

At the time, the design represented the maximum power obtainable from a ten-coupled non-articulated locomotive which was limited to a 15 long tons (15.2 tonnes) axle load on 60 pounds per yard (30 kilograms per metre) rail.

To enable it to negotiate tight curves of 300 feet (91 metres) radius, the third and fourth coupled wheelsets were flangeless.

Its 5-inch thick (127-millimetre) bar frames extended from the front to the hind buffer beams and were stayed by cross stretchers of a light fabricated construction.

[1][2][3] The result could be considered as Watson's answer to the Class GF Garratt locomotive, the two types having very similar axle loads.

Thus, in the year following Watson's departure, the Class 21 locomotive was delivered with Walschaerts valve gear.

On the Class 21, the difference in stroke was made up by providing deep spigots on the cylinder covers.

To distribute the axle loads to within the maximum allowed for branch lines, the ten-coupled arrangement with a four-wheeled trailing bogie was required.

To further assist with keeping within the permissible weight limits, the cab was of welded construction and all footplates and running boards were made of duralumin while the axles and crank pins of the coupled wheels were hollow and the connecting rods were of special high tensile steel.

[3] The locomotive began its career on trials at Braamfontein in 1937, working the Rhodesian Mail trains between Randfontein and Mafeking.

[2][3][10] At the time, it seemed that ten-coupled engines could not be used as general utility types suitable for the rather severe curvature which existed on SAR lines, particularly those with rail of 45 pounds per yard (22 kilograms per metre) and 60 pounds per yard (30 kilograms per metre) where the curvature was generally more severe than elsewhere.

The proposed Class 22 was also to have a 2-10-4 Texas wheel arrangement, but was to have been a heavy mainline version of the Class 21 with an axle load of 22 long tons (22.4 tonnes), the heaviest that current SAR track could bear on its 96 pounds per yard (48 kilograms per metre) mainline rail.

The proposed boiler pressure was 250 pounds per square inch (1,724 kilopascals), a figure never attained on the SAR, and its anticipated tractive effort of 66,406 pounds-force (295.39 kilonewtons) at 75% of boiler pressure would have made it capable of handling loads of 2,200 long tons (2,235 tonnes) on the coal run from Witbank to Johannesburg with comparative ease.

Another factor which acted against the project was the insufficient length of the receiving sidings in the yards which made it doubtful that such a locomotive would have been able to be used to its full capacity.