The South African Railways Class 21 2-10-4 of 1937 was a class of steam locomotives used in South Africa. In 1937, the South African Railways placed a single Class 21 steam locomotive with a Texas type wheel arrangement in service, designed as a mixed traffic locomotive suitable for use on light rail. A simultaneously proposed heavier mainline version Class 22 was never built. Manufacturer 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. It was built by the North British Locomotive Company in Glasgow and delivered in 1937. Only one locomotive was built, numbered 2551. At the time, the design represented the maximum power obtainable from a ten-coupled non-articulated locomotive which was limited to a axle load on rail. Characteristics Watson disliked articulated locomotives and his aim was to develop a powerful non-articulated mixed traffic branch line locomotive with an axle load suitable for light rail. To enable it to negotiate tight curves of radius, the third and fourth coupled wheelsets were flangeless. In addition, a total side play of was provided on the leading coupled wheels and spherical bearings were fitted to the leading coupling rods. Its bar frames extended from the front to the hind buffer beams and were stayed by cross stretchers of a light fabricated construction. It used a Watson Standard no. 3B boiler, the same as that used in the Classes 15E, 15F and 23. The end result could be considered as Watson's answer to the Class GF Garratt locomotive, the two types having very similar axle loads. The Class 21 carried more water than the Garratt and was about heavier with tractive effort, compared to the of the Class GF. Even so, only one Class 21 was produced and the design was not repeated. Watson's design called for cylinders with rotary cam poppet valve gear, but since the locomotive was still under construction when Watson retired, his successor as CME, W.A.J. Day, made use of the opportunity to alter the specifications. Thus, in the year following Watson's departure, the Class 21 locomotive was delivered with Walschaerts valve gear. The cylinders were interchangeable with one another and with those of the Classes 15F and 23, even though the piston stroke of the Class 21 was compared to in respect of the other two classes. On the Class 21,