Looks like we both had incomplete information. I didn't know that some trains have their doors really close to the ground and it's actually very inconsistent. I'm getting a clearer picture now.
It is at least consistent within any one Amtrak route. The ones that go through the north east have to go through tunnels that can only accommodate single–level cars, while all the other routes have the double–level cars where the doors are really close to the ground.
