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 odd engines might be seen almost anywhere in the GWR area.
The 56xx class was developed from the later 0-6-2 tank engines which had proved so successful for the Taff Vale and Rhymney railways and where additional locomotives were needed for the South Wales coal traffic. These are powerful engines for their size, rated 5MT by British Railways and found useful employment across the GWR network. 200 examples were constructed between 1924 and 1928, with withdrawals not commencing until 1962 following the delivery of the first Hymeks.