In 1911, the Central South African Railways placed nine Mallet articulated compound steam locomotives with a 2-6-6-2 wheel arrangement in service.

They were erected at the Durban workshops in March 1911 and were very similar to the experimental Class MD which had been delivered in 1910, except that the new locomotives were equipped with Schmidt superheaters.

By comparison, in the more usual arrangement of simple expansion (simplex), steam is expanded just once in any one cylinder before being exhausted through the smokebox.

Their spent steam is then fed to the larger low-pressure cylinders which drive the front set of coupled wheels.

[1][3] The most noticeable external difference from the experimental Class MD engine was the altered arrangement of the main steam pipes necessitated by the superheater.

While the locomotive was brought to the Transvaal, difficulty was experienced at Drummond and Laing's Nek where the drain angles on the cab roof struck the sides of the tunnels.

1620 was used in a test run from Witbank to Germiston, hauling 55 bogie wagons with an all-up weight of 1,626 long tons (1,652 tonnes).