I found this in the Illustrated Treasury of the MLW - it shows a view of the bottom of the frame of a C630/M630 being built at the plant in Montreal. There is indeed a lot of space between the bottom of the "visible" frame (walkways, etc.) and the actual bottom of the frame itself (where the fuel tank is mounted). The height perceived on the Bowser loco is prototype, it's just that Bowser didn't fill it in like the 1:1 folks did - hence the see-through space underneath. In the background one can see one of the trucks waiting.