I had no problem getting satisfactory joints, but with so many small parts so close together, I had to use a lot of clamps to keep nearby joints from releasing. Here are a few pics of the bridge.Don F;
I checked your website and got the e-mail address. I also saw that you are modeling in O-scale. If that is so then you're going to be using some big parts. (I'm in N-scale) I'm not sure that my unit would produce enough heat to do what you want. However there's no reason you couldn't build a higher power model. I have had little success soldering steel. I've heard of it and even tried it with steel wire for my catenary. However the steel never seems to form a strong permanent joint. The joints once made, tended to come apart fairly easily. I have since switched to brass poles and phosphor bronze wire, both of which solder extremely well, forming very strong, and permanent, joints.
I'll be happy to e-mail info to you, but have you considered/rejected some sort of small arc welding tool? I think it might be able to produce much better joints in steel, than soldering.
I would appreciate you sending the how to info for the resistance soldering rig.