When the smaller railway companies were forcibly merged into the GWR, these modern 0-6-2's were in generally good order (some were a few months old) and had proved successful.
Included in Collett's innovations was a standard number 2 boiler which was suitable for the 5600 (and the M and R class Rhymney locomotives), complete with the traditional copper GWR safety valve casing and copper-capped chimney.
Some A and P classes were also rebuilt but used the slightly shorter standard number 10 boiler, also to good effect.
This was because the nature of the work they undertook demanded high adhesive weight, plenty of power with good braking ability, but no need for outright speed, nor large tanks or bunker as the distances from pit to port were short.
All the Rhymney Stevenson derived locos passed into British Railways (BR) ownership in 1948.