I have a first gen Shay and its like 8 inch clearance as I recall. Is that the tallest bachmann? Working on an over/under and trying to decide clearance. Doubt I would ever run anything like the Big Boy and not even sure on its height requirements.
Mickey,
The best advice I can give is to ALWAYS allow 10" free clearance from top of rail to the underside of the lowest overhead obstruction and you will not have any issues.
As for the Big Boy, you are comparing apples and bananas. The shay is a 1:20.3 model of a narrow gauge locomotive, the Big Boy is a 1:32 scale model of a mainline engine. Just the scale difference will tell you the Big Boy will have a smaller overhead clearance. If you can run a 1:20.3 model through it a 1:32 will clear (height wise).
Bob C.