Although these coaches were in production for a relatively short period they came to be as much recognised as 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 toilet 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 the later builds had rounded corners to the windows. The square cornered version of this diagram is available as Comet code E2K.
The coaches entered service on premier routes on the LNER. Interchanging of stock in Scotland led to them being found on the West Coast main line also.