How detailed would you like the model, LX?
If it was going to be used only in the distance shots, one could bake a lightweight one using a texture map for most of the girders… however if you were going to do closeups of birdnests in the girders, fights in the beams, car chases between the pylons, etc then you’d probably want the near ones (at least) to be fully modelled including every rivet (to say nothing of the actual rails on top and the sleepers… high-poly model sufficient for tying people to the tracks? 
).
Devil’s in the details X).
