Good morning folks.
Carl - There is 8.5 inches of clearance from the top of the board to the ceiling. Add in the 0.5-inch cork board and the clearance is 8 inches which is where my peace of mind threshold will be for the 5.5-5.75 inch high husky stacks and auto carriers. The bridge information MTH provides is all over the place (It lists some 2-track bridges having the same dimension as single track bridges). Says that the bridge is 14 inches high but that includes the supports.
I'm going to have to go back to the LHS (Ready to Roll Trains as my train is pictured below) and measure the dimensions of one right out of the box for myself, not what MTH says it is, unless a fellow OGR member has the dimensions handy 
I don't want to have to resort to a girder type bridge - also in the above photo - as those are too short in length and I would have to jerry rig with steel rods, etc. so that the connection of the two bridges is properly supported. Will have to do so some internet searches for O Scale bridges that will fit my requirements. Atlas O's 6923 Deck Girder Bridge would be great fit and look but is ridiculously expensive, and as it's a single track, I would have to buy two of them. Having a single track bridge for a 2-track set up would be great for a layout in which one is trying to simulate realistic operations with one train having to wait for the other to clear, etc., but that works on a surface that's at eye level or helicopter view. What would not work really well if you're going to have to crane your neck upwards constantly to do that. Ceiling layouts, are for the most part, designed as loop runners. Scratch building a bridge is not an option.
Mallard + RSJB18 - Understood, and it's not too late. The two boards that you see in the photo is all that I've purchased so far just for testing purposes. Haven't invested heavily yet in lumber but I do know that I won't be using the MDF because its too heavy. Interesting idea on gluing the 1-inch pine to side of plywood for the clean surface.
