A new set of 'standard' locomotives was to be built by British Railways, based on LMS designs and incorporating modern ideas.

In particular, the Standard design incorporated features designed to make disposal of the engine after a working "turn" easier: a self-cleaning smokebox and a rocking grate removed the necessity for crews to undertake dirty and strenuous duties at the end of a long shift.

The most obvious visible changes were a higher running plate, slightly enlarged driving wheels (from 6 ft 0 in (1.829 m) to 6 ft 2 in (1.880 m)), increased cylinder bore (from 18½ in (457 mm) to 19 in(483 mm)), a standard cab with external pipework and the regulator gland on the driver's side of the boiler below the dome.

Like the "Clan" class locomotives, the Standard 5s, with their high-stepped running board, were partly conceived to be more economical and serviceable replacements for the Bulleid Pacific.

They began reaching full potential with different firing techniques to let them to steam using poor quality coal were developed.

Another proposal was to produce freight locomotives based from this design in response to criticism by the Western Region on the BR Standard Class 9F.

The management believed that the 9Fs were too large and powerful for most heavy freight traffic, along with being far more expensive to build and operate than the older 2-8-0 locomotives they supplemented.

Changes included a higher boiler pressure of 250 psi and driving wheels 5 ft 0 in (1.52 m) in diameter to increase tractive effort.

73050 was purchased directly from British Railways for preservation while the other four engines were rescued from Woodham Brothers scrapyard at Barry Island.