DrDan, I hate to throw cold water on a hot fire, but unless I'm reading things wrong, you are headed for some trouble with a bridge crane. First, alignment of the rails and bridge is critical, or the bridge will crab and lock up. The jet trollys would give you trouble, unless you double up on them and but a 4 foot beam between them.
The big question in my mind is do you need as much crane as you are engineering? Wouldn't you be able to use a double A frame with a beam across the top and a good set of casters on it to accomplish your goal? I've built a couple of that style and find them far more useful than a bridge since they can be moved in and out of place relatively quick. We have no trouble moving them with 1800# hanging on the 8" casters on a concrete floor.