Although these coaches were in production for a relatively short period they came to be as much recognised as were the earlier Gresley designs. They had steel flush sides, straight roof, an internal arrangement whereby no passenger had to walk past more than two compartments to reach an exit door, but in a reversion to earlier practice on this and other railways, no cross vestibules at the ends of the coach. The graceful oval windows were distinctive. Initially all Thompson designs followed Gresley practice in having square cornered windows, which led to continual problems of rusting in the bottom corners. In an attempt to combat this these particular coaches were modified by welding a thin metal strip around the window apertures, and the etch portrays the coaches in this condition. Later designs followed contemporary practice on other railways in having rounded corners to the main windows.
These coaches were built at York initially for the ECML but in 1950 they were re-numbered and allocated to ‘all line’ services. Interchanging of stock in Scotland led to them being found on the West Coast main line also.