> Double-decker cars are significantly more common in continental Europe.

And west of Chicago in the US. Amtrak runs double-decker cars west of Chicago and in fact Chicago's passenger rail service (Metra) is mostly (all?) double-decker.

My understanding is that tunnel clearance in the east is not sufficient for the double-decker cars.

I also see containers stacked two high and auto carriers that appear to be similar height and these probably also don't travel east in the US for the same reason.