I recently installed some girder bridge sections to span the main line East, and also what will be a two or four lane road. I spray painted the sections a flat gray, then used some Pan Pastel powders for some weathering. I will add more weathering when I do the scenery around that area. For comparison, the second picture shows a closer view where the left section is weathered, and the right section is not. The bridge abutment under the track is temporary, and will be replaced by something with more visual interest.
Andy