Standard domes are shorter than stacks, tri-level auto racks are also shorter than double stacks, there are also two classes of double stacks, overseas shipping containers are shorter than domestic containers like J B Hunt and BN America containers. Domestic containers are the tallest combo and currently that is the standard used to determine clearances. This prevents accidents from happening like happened a few years ago on the UP between Spokane and Hinkle where overseas doubles fit, but when they ran a BNSF detour they packed a tunnel full of nice fully loaded J B Hunt containers--took a couple of days to unstuff that mess and reline the tunnel timbers---one container had beer in it and the track gangs had some nice times salvaging.