I would HIGHLY suggest to NOT use the RR steel for a bridge. I have looked into that before i used a culvert to make my stream crossing. The RR steel is carbon steel and is NOT made for free weight support. It will develope cracks midline and eventually split. Its fine for a walk bridge but will not support a bunch of weight without support under it , just like a rr bed with the ties. Had DNR out here to advise me on this issue. Had them out when i 1st bought the place to see if any monetary things were available to help reclaim the forest and make it look better. Been working closely with him whenever i have any questions, ie trees to keep....bridges/culverts/stream crossings etc.
The RR steel has very little strength by itself and i would reconsider this idea, or at least do some HEAVY research into the structural strength of it comparfed to the load you plan on putting on it. Consider any heavy trucks that may deliver material to your place in the future as well. Dont forget the weight of a cement truck in the future. Who knows, you might need one for future projects. advice.....DONT USE THEM FOR A VEHICLE BRIDGE!!!!!!!!!!!!!!!!! (at least without support under them) they look and feel strong and heavy, but are weak compared to thier weight.