Just a guess, but it may have something to do with vehicle shape.

A train has straight sides and is just as wide at the ceiling level as it is at the floor level. An airplane -- not so much.

While you may be able to get 5 across seating at floor level, you might not be able to do that 5 feet above floor level.

If you sleeping berths in an airplane like they sometimes to on trains, I would imagine that those in upper level compartmetns would have to be shorter than those in lower level compartments.

Cromely.