Here are pictures of the MTH version from a 2007 release. All releases so far have been the same dimensions with only 40ft containers on the bottom with ability to put 48ft containers on the top. The dimensions in the new online catalog list the exact same dimensions of the cars as the previous releases.
If you look at the pictures, it is obvious that the wheels sit up into the well area higher than the bottom of the well, thus you can’t have a 48ft container on the bottom.
I just happened to have these out on the layout on a train sitting next to another with 40 and 48 containers on it.