The class 15E was designed by A.G. Watson as a further development of the class 15C Mountain for mixed service. At that time, in 1935, Watson introduced several standardization features for the classes 15E, 16E and 19C, which had been ordered simultaneously. All of these classes featured rotary cam poppet valves. The 16E Pacific and the 15E Mountain had the same 24-by-28-inch cylinders. The Mountain was equipped with the Watson Standard 3B boiler, in contrast to the shorter 3A boiler on the Pacific. Both had virtually the same firebox with a grate area of 63 square feet, 206 square feet in the walls, and 26 square feet in arch tubes.

The first 20 were built in 1935 by Robert Stephenson & Hawthorns. In the following year, Henschel built 16 and BMAG built eight. The German locomotives were three-quarters of a ton lighter. They were nicknamed “Bongols” and initially hauled the heaviest and most important trains. Soon, the Class 15F was built in larger numbers with Walschaerts valve gear. In the 1950s, both were pushed into secondary service and replaced by the classes 25 and 25NC. The last ones were taken out of service in 1973, though some were sold to Rhodesia or Mozambique. 2878 is preserved by the Transnet Heritage Foundation in Bloemfontein.