The end walls of each compatment are 1x3x4 panels, with bench style seating half-studded into the recess, this gives 1 stud footroom. It also means a very uncomfortable sitting position for the passengers, as the top of the panel is at the back of their heads, and the have to lean forward to remain seated.

I'm not entirely happy with the flat ends, and have devised a second base in which the centre 4 rows of studs are at a half stud offset, and extended to be 19 studs long. this will allow me to make the end walls from normal bricks, while still keeping the bench seats and 1 stud legroom.

Incidentally the only reason I used the BR 7720 doors was due to a parts shortage ;-)