Rick,
You'd run into the same problem (coupler height) because none of the manufacturers have agreed on a "standard" height for anything.
We would need Lionel, MTH, Atlas, WBB, Weaver, 3rd Rail, and whoever else I've left off (and you still have to deal what's already out there) to agree on something like a standard height for the bottom of the floor, or as you noted, the end sill. In other words, a SINGLE point of reference.
What we have with Kadee couplers IS a single point of reference, 25/32" from the top of the rail to the top of the coupler box.
