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.North British Locomotive Company works list, compiled by Austrian locomotive historian Bernhard Schmeiser
