In 1925, the South African Railways placed two American-built Class 15C steam locomotives with a 4-8-2 Mountain type wheel arrangement in service.

They introduced several features which were new to the SAR at the time, such as top feeds to the boiler, self-cleaning smokeboxes, Sellar's drifting valves, and grease lubrication to the coupled wheel axleboxes, crank pins and connecting rod big ends.

Their Type KT tenders, arranged with self-trimming type coal bunkers, had a coal capacity of 14 long tons (14.2 tonnes) and a water capacity of 6,000 imperial gallons (27,300 litres), which was considered sufficient for the engines to travel 100 miles (161 kilometres) without having to replenish water.

To reduce the length between boiler tube plates, a 2 feet 3 inches (686 millimetres) long combustion chamber was provided in the firebox.

They performed well, were comparatively trouble-free and were good steamers capable of handling a heavy load exceeding that of any other locomotive which had been employed on this section to date.

All twelve engines eventually had their coupled wheels retyred to 60 inches (1,520 millimetres) diameter and, to compensate for the loss of tractive effort due to the larger diameter wheels, their operating boiler pressure was increased to 210 pounds per square inch (1,450 kilopascals) by an adjustment of the setting of their safety valves.

[5][6] The locomotives gave good service and were justifiably popular with traffic, running and engineering staff alike.

The introduction of these large engines along with their Class 16D Pacific type Big Bertha siblings resulted in a complete revision of traffic operation between Cape Town and Johannesburg.

Following electrification of the Eastern Transvaal mainline in the mid-1960s, commencing in December 1964 all twelve Class 15CBs were transferred to Greyville on the Natal System from where they were used on, amongst others, the night and day passenger trains to and from Empangeni on the North Coast line and on passenger trains to and from Kelso on the South Coast line.