re: OB Engineers: Steel Beams to Make a Bridge

It sounds like you've moved on from wanting to span the 75'-0" with two beams in parallel (with the beams spliced somewhere in the middle), but just in case you're still thinking about it here's a little info. There's no such thing as a W12x55 so assuming it's a W12x53 and is fairly old (A36 steel)

A W12x53 spanning 75'-0" considering only the weight of the beam (53 lb/ft) and a 10,000 point load at the midspan (assuming 20,000 lbs split between two beams) the beam will be at 160% capacity and deflect 15.5 inches in the middle. This doesn't take account the weight of the bridge driving surface which will further exasperate the problem.