Between 1954 and 1958, the South African Railways placed 120 Class GMA Garratt articulated steam locomotives with a 4-8-2+2-8-4 Double Mountain type wheel arrangement in service.

These first 25 locomotives were equipped with Type X-17 water tenders, built by the SAR in its Pietermaritzburg shops in 1953.

[1][3] Some of these locomotives, intended for working in areas where there were tunnels such as on the East London mainline, were initially equipped with steam-operated smoke deflecting cowls over their chimneys.

The first batch of 25 locomotives were tended by the same Type X-17 water tender which was used with the Class GM, with a capacity of between 6,750 and 6,815 imperial gallons (30,700 and 31,000 litres).

The rest of the locomotives were tended by Type X-20 water tenders with a capacity of 6,790 imperial gallons (30,900 litres).

The second and third batches of locomotives were ordered from 1956 to assist with moving large volumes of traffic, mostly coal, from the Transvaal to destinations in the Free State and Cape Province.

Until the Class DE-1, the SAR's first road diesels, took over this task late in 1958, they were employed on block coal workings from Witbank to Kroonstad, a distance of 208 miles (335 kilometres).

[17][18] In Natal, the bulk of the Class was based at Pietermaritzburg, from where they worked most trains on the two heavily graded branch lines to Greytown and Franklin.

This turned out to be their last term in mainline service since they were replaced by Class 25NC locomotives in 1984 when the line was relaid with heavier rail.

[4][5] Towards the end of their service lives, the Eastern Transvaal system still had an allocation of them, where locomotives from the Waterval Boven and Breyten sheds worked the line down to Vryheid in Natal.

During the period from August 1979 to September 1981, altogether 26 locomotives of the Class were hired to Zimbabwe-Rhodesia, but not all at the same time since they were rotated with Capital Park in Pretoria as their nominal home for the occasions when they had to return to South Africa for major repairs.

Since the Rhodesian bush war was still ongoing in 1979, the locomotives were equipped with armour plating around the cab.

In an attempt to solve this problem, RR extended the height of the Class GMAM's coal bunker by a foot to increase the capacity.

Two were sold to the Hotham Valley Railway in Western Australia where they were to haul tourists in ex SAR passenger coaches.

These two locomotives, 4090 and 4129 however, never left South Africa and after standing at Bloemfontein loco for 20 years, they were scrapped in May 2016.

4074 was rebuilt after REGM service with the boiler cradle off 4126 and therefore is shown listed as such per the Beyer Peacock numbering and locomotive identification protocol.