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.
These carriages had square cornered windows, which rapidly corroded in service. 2″ wide beading (Comet C61) was fitted as a repair. Subsequent builds to the same diagram (Comet E18K) had round corners to the windows to avoid corrosion.
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.