These powerful tank engines were a development of the pre-grouping Taff Vale and Rhymney Railways' locomotives, designed specially for hauling empty wagons up the valleys to the pits and controlling loaded trains on the way down to the South Wales ports. The 56xx proved so useful 200 were built and allocated all across the GWR network, a powerful and capable goods tank engine also able to haul local passenger trains when needed.
First introduced in 1924 as Collett's design for the welsh valleys, the 56xx class was later classified 5MT in BR days. Powerful and sturdy they were mainly to be found in Wales and the West although these engines might be seen almost anywhere in the GWR area.