It was essentially a hybrid design, the chassis being closely based on and sharing a number of parts with the LMS Ivatt Class 4, and having a boiler derived from a GWR No.2 boiler as fitted to the GWR Large Prairie 2-6-2T and 5600 Class 0-6-2T tank engines.

Strangely the class did not share the same design of wheels as the Doncaster-designed BR Standard Class 4 2-6-0, however both shared the 5 ft 3 in (1,600 mm) driving wheels and the same piston stroke, and hence crank-pin throw.

The cylinder covers of engines as built were fitted with "screw-in" type pressure relief valves.

From September 1955 revised cylinder covers were introduced for renewals incorporating "bolt-on" type pressure relief valves.

None were preserved, though some components were saved from scrapyards for use on a project to build a new Standard Class 3 2-6-2T.