Make sure there are trains on them…
I could easily understand someone’s concern if they checked your images and found nothing but bridges, unless you’re an engineering student…
I am in the middle of a long-term project to photograph all the railroad bridges crossing the Colorado River in Texas. To this point, I have never had any issues. In smaller towns, I often find the Chamber of Commerce office and ask about the bridge and the town. It is surprising how many small town Chamber offices are in unused RR stations. I had alway received a lot of information and cooperation.
I also have used all the internet mapping sites to plan my route to a specific location for a photo. It seems to look a lot better to authorities (and concerned citizens) if you look like you know where your going, that if you are just bumbling around.
Keep safety in mind at ALL times and be aware of your surroundings. With photography, it is easy to get focussed on the subject an lose awareness of where you are.
Finally it is very rare to find trains on some bridges. I have one photo of the first train that went over the old SP bridge at Kingsland Texas in over two years. That takes a lot of research and waiting. Also luck. In this case I got the photo because my granddaughter needed a diaper change and the park at the end of the bridge was the most convenient pulloff.
dd