When developing a class 5 standard locomotive for mixed service, a Pacific locomotive first came up for discussion. However, since this was considered too expensive, the choice fell on the wheel arrangement 4-6-0. This offered the additional advantage that, for a given total weight, a larger proportion was accounted for by the adhesive weight. The basis for the development was Stanier's LMS Class 5, which had proven itself in all service areas and was still in production at the time.
The improvements still to be made to this mainly concerned the production and operating costs, which were to be reduced through the use of standardized parts and the simplest possible maintenance. A self-cleaning smoke box and a rocking grate were installed for faster turnaround after the end of the shift. The cab was standardized with many similarities to the other standard classes. Some pipes in the cab have been moved to the outside for easier access and cheaper production. The boiler had almost the same dimensions as the Black Five, but was made of manganese steel instead of nickel steel. In addition, the drivers were enlarged by two inches and the cylinder diameter by half an inch.
The production of the standard locomotive followed the production of the Black Five almost seamlessly. Between April 1951 and January 1952 only 30 were initially built in the workshops at Derby. A further 100 were later built at Derby and 42 at Doncaster, with the last being delivered in June and May 1957 respectively. The locomotives were used in many regions and were just as versatile as their predecessors. They were reported to have been more economical at higher speeds than the Black Five and the drivers would have credited them with a top speed of just under 100mph.
Variants with a double blast pipe and a 2-8-0 goods variant were proposed, but these were not implemented. This was mainly due to British Railways' modernization plans from 1955, which prepared the phase-out of steam traction. The locomotives were retired between 1964 and 1968 and today there are five of which two were still operational.