In 1935, the South African Railways placed one Class 20 steam locomotive with a 2-10-2 Santa Fe type wheel arrangement in service, designed and built at its Pretoria Mechanical Shops.

The locomotive was intended for use on the South West Africa system, where the tracks consisted of 40+1⁄4 pounds per yard (20 kilograms per metre) section rail laid in desert conditions and practically without ballast.

[1][2][3] Even though it was not wholly a South African product with its imported frames and cylinders and its inherited boiler from a Swiss-built locomotive, this was the third recorded instance of steam locomotives designed and constructed in South African workshops after the Natal Government Railways' engine Havelock of 1888 and the Class 2C of 1910.

The total weight of the engine and tender in full working order was 124 long tons 8 hundredweight (126,400 kilograms).

[1][2][3][10] Photographs show that, after being returned to Pretoria, the engine's Type MP1 tender was replaced by a larger Type MT2 tender with a 14 long tons (14.2 tonnes) coal capacity, a 6,000 imperial gallons (27,300 litres) water capacity and a 17 long tons 15 hundredweight (18,030 kilograms) axle load.

The total weight of the engine and Type MT2 tender in full working order was 142 long tons 15 hundredweight (145,000 kilograms).

The Pretoria enginemen considered the Class 20 to be one of their best locomotives since it was free-steaming, more than usually trouble-free and able to handle any load they gave it.

Such locomotives had by then already been built by Henschel and Son for use in Argentina, 240 Kriegsloks for the Eastern Front in Germany in the 1930s and more than 4,000 units of the Russian SOK class, mainly for use in Turkestan and other arid regions of the Soviet Union.

The total weight of the modified engine and Type CL tender in full working order was 158 long tons 3 hundredweight (160,700 kilograms).

[1][3][10] At the end of 1951, the locomotive was relocated to Touws River where it was used for further condensing tests and occasionally put to work in regular service.

2485 was again relocated, this time to De Aar for service on the section via Prieska to Upington on the line to Windhoek in South West Africa.