During 1938 and 1939, the South African Railways placed sixteen Class GM Garratt articulated steam locomotives with a 4-8-2+2-8-4 Double Mountain type wheel arrangement in goods train service on the Mafeking line out of Johannesburg.

[1][2][3][8] The boilers were of outstanding proportions to enable the locomotives to cope with one particular bank on the Zeerust line which required continuous steaming for 75 minutes.

The firebox was fitted with two Nicholson thermic syphons to increase water circulation and also to provide additional heating surface.

[1][2][3] Their type H.T-1 mechanical stokers were capable of delivering 15,000 pounds (6,804 kilograms) of coal per hour, which ensured a high firing rate when required.

The stoker was driven by a totally enclosed double-acting two-cylinder variable-speed reversing steam engine, located on the hind pivot casting and operated from the cab.

The injector feed pipes led to a combined top-feed check valve, situated on the first barrel plate on the top centre-line of the boiler.

Soft grease lubrication was used throughout for the motion gear, except the piston rods, valve spindles and main crossheads which were oil-lubricated.

The front pivot was a spherical type, located on the front engine unit's frame between the driving and trailing coupled wheels, while the rear pivot was a flat oil-bath type, located on the rear engine unit's frame over the driving wheels.

The Class GM remained working on the Mafeking section, initially out of Johannesburg and after electrification of the West Rand lines out of Krugersdorp, until they were replaced by diesel traction in 1972.

[3][4][5][11] Three of them were soon placed back in service when a series of accidents in the Eastern Transvaal led to a shortage of Class GMAM Garratts.

They remained in extended service for more than a year, initially taking part in the mainline workings to Piet Retief and later employed on short pick-up trips to Ermelo and on the Spitskop Colliery shunt.