As many components as possible from the original W1 were used in the rebuild, including the frames, which were shortened by 1½ feet at the front end. Other items, where needed, were sourced from the A4 class, the A3 class or, like the front bogie, from the B17 and an experimental double Kylchap blastpipe and chimney was fitted. The cab, while similar to that of the A4 class, was considerably longer and this led to the retained original tender's coal delivery chute having to be lengthened to compensate for the extra distance from the firebox doors. The original tender was retained, but with superficial modifications, such as to the curvature of the front plates and the addition of streamlined plating to the front and rear, although this was partially removed just a year later, in 1938. Up to the outbreak of war in 1939, and during the war years, 10000 worked mainly on Kings Cross main line duties and with its power and smooth riding it was a popular engine with the LNER's drivers, though less so with the firemen due to the distance between the shovelling plate and the firehole door.