 Converted Swing Door cars originally entered service with class codes such as 'AT', 'BCM', and 'ABCD', indicating both class and type.In 1921, that was largely simplified to 'M' (Motor car), 'T' (Trailer car) and 'D' (Driving trailer), the trailers being first class and motor cars second class, with some exceptions.The maximum size of the Swing Door train fleet was