Sir Nigel Gresley's W1 4-6-4HP 10000 entered Darlington Works on 21st August 1935 for what was anticipated to be a major overhaul and modification. During this period Gresley decided to rebuild 10000 as a three-cylinder locomotive with a conventional firetube boiler. It was moved to Doncaster Works on 13th October 1936.
When 10000 was finally released from Doncaster Works in November 1937, it was a significantly different locomotive; fitted with a diagram 111 boiler that was very similar to the diagram 108 boiler fitted to 2006 ‘Wolf of Badenoch’. It also had a streamlined casing to the same pattern as the A4 class.