Had a buddy who did the same thing with an old, decommissioned (18-wheeler) flatbed trailer. Hauled it out to the creek crossing, backed it across, dropped it, shoved it into place with a dozer, pulled the wheels/tires off of it, built an abutment with some logs, pushed some dirt into both ends and had a perfect bridge that you could drive a D-9 Cat over if you wanted to. Makes a really slick bridge and I would think a flatbed trailer would be much cheaper to buy and transport than a rail car.

Makes more sense really as you can pull it to wherever you want it, then simply pull the wheels off once you get it set up.