First, is it the Rust, Silver, or Black one?
We have left out the center pier as it just doesn't seem right to have a pier under the arch center, and used a black wash on the end two. Made a few spots where there might be a small bit of weeds, small surface crack lines, and sadly graffiti if you model current. On the bridge we used dust as dirt and then some downward streaks of rust, (think the silver would be the easiest to overdo the rust weathering on, the black one the dirt) and exhaust marks down the center. Most heavily used bridges around here the railroads seem to take pretty good care of with regular maintenance and paint.