After the 1923 grouping, Swindon inherited a large and varied collection of locomotives from historic Welsh railway companies, which did not fit into their standardisation programme.

GWR boiler inspectors arrived en masse and either condemned the original locomotives or had them rebuilt.

Two hundred GWR 5600 Class locomotives were built and remained in service until withdrawn by British Railways between 1962 and 1965.

A number of them passed into British Railways (BR) ownership in 1948, including (with some gaps in numbering): Of the Taff Vale Railway, many engines continued to operate up to the 1950s, but today only two locomotives survived, TVR 'O1' No.28, the last-surviving Welsh-built engine, and TVR 'O2' No.85.

The 5600 Class was specially designed for work in South Wales, replacing the worn-out locomotives that were taken over from smaller railways in 1923.

Contrary to this trend, the Rhymney Railway's more modern 0-6-2s were in generally good order and had proved successful.

They also had the tendency to derail, so those driving them preferred them in reverse, where the pony truck was able to guide them around tight curves.

This resulted in some minor design differences from the Swindon locomotives, most visible were additional balance weights fitted inside the driving wheels webs opposite the crank pin to remedy the faults.

Due to the stability of the design, many drivers would typically operate the 56xx class down the Welsh valleys in reverse (bunker first).

As the locomotives were operated mainly in South Wales, some railfans know the Class by the nickname "Taffy Tank";[12][13] 'Taffy' being a derogatory term for someone of Welsh descent.