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.
Double decker trains I just find fun for some reason, and you get slightly better views riding up top over fences and walls and such.
I remember taking the bus from Plymouth to Millbrook (mid 1990s), which included a trip over the River Tamar on the ferry. I can't remember if it was one bus or two to get there and back.
One year I recall a harsh winter and icy roads. On one trip, I was sitting in one of the top front seats. It was early evening and I was heading home to Plymouth, idly watching the stiff, whitish countryside through the haze on the windows. The bus lost grip on a gentle incline and started to spin gently but, well, I nearly needed to change my trousers. We did a good two complete spins until a wheel hit a bank and we came to a halt facing mostly backwards. "Drives", made a polite announcement and reversed down to the bottom and then did a very gentle 10 point turn and cracked on.
Fair enough - we are missing out on some lovely views.