It's the width of a Roman Chariot - what's wrong with that 8) It was the first and that's the way it goes.

Not sure why you need a double decker train and I think you could if you really packed people in. You can just make them twice as long. Railways don't have the same constraints as nearly anything else.

I remember the carriages of the later half of C20 and each person had a huge amount of space. In one design you had a corridor running the length of the carriage with a series of bays with seating for up to four or five on each side, loads of leg room and a luggage rack above. Separate door to the platform for each bay. Obviously you had to puff furiously on your pipe or fags to fill the space with healthy smoke.