Pete, I have had good success with replacing every 6th tie or so with PCB ties and soldering the guard rails to them. This is very easy when you are laying the track, but I don't see how this would be possible to do working through your existing bridge trusses. Epoxy may work if you can sand the top of the ties and bottom of the rails to give it a better grip.
Why not cut the long rail into sections, and just glue them butted together with three thicknesses of paper between? Remove the paper and you probably wouldn't notice the gaps anyway. You could cut them into 40 foot lengths. Split rail joiners in half long ways and solder one half of one of these halves to the inside of a guard rail and let the other half stick out as a guide for the next guard rail. The glue will probably hold on the shorter length that isn't likely to exceed the elasticity of the glue, or use "Liquid Nails for Crafts", which remains slightly flexible. When painted, the joiners (that you could see), would look realistic, hold the rails in alignment, and be easier to install, since the glue would go along the bottom of the rail anyway. The other half of the joiners you cut off are not there to present a problem between the guard rail and running rail.
