India does double-stacked trains now.
They're one of the tallest in the world as they use flatcars instead of the well cars used elsewhere. The broad gauge allows for the extra stability necessary for double stacking without lowering the center of gravity
https://www.youtube.com/watch?v=jiMqNrSUZ0Q>>1393291BART was designed by aerospace engineers with no experience in railway engineering. They chose 5'6" because it theoretically allowed for better ride quality.
In addition, they used flat edge rail and cylindrical treads causing flange squeal. This resulted in it being one of the loudest transit systems in the world, with ear-damaging volumes being commonplace.
The sound was even used in the Dead Space horror games as it's so eerie.
