I can understand your concern. This is going to be difficult for two reasons:
- Finding out the agency that is responsible for the maintenance may be next to impossible for a layperson. Note that I said "maintenance", because this might be a different entity than the owner due to agreements.
- Due to the number of complaints that the public agencies receive, it is very likely that your comment will get ignored. This often has nothing to do with whether or not your concern has merit.
Public vs. Private
Public agencies will be the easiest to contact. In a city, calling the city emergency or non-emergency number will at least let you talk to someone who could forward your concern on to the appropriate agency. What is done after that is uncertain unless there is an active emergency situation, e.g. bridge span missing. In that case, the standard emergency procedures will be in effect.
A simple concern will likely get ignored. At the best a local worker will be dispatched in the next day or so to do a drive-by inspection.
Private agencies might not have an easy method for interfacing with the public. Depending on the company, the headquarters might not be located in the same state as you. I am specifically thinking about Railroads in this instance. To find a contact number, it might involve sitting down and searching a website.
Once again, active emergencies will be handled by the local emergency (911) management. Simple concerns will likely be ignored.
Every agency (private or public) that I can think of has inspection requirements. These regulate the frequency and extent of inspections. For many typical bridges, an inspection is required every year. These are usually routine inspections which mean that at a minimum every area is visually inspected. Certain structure of concern might have more frequent requirements or more in depth inspections. These inspections might include actually measuring the thickness of steel plates or sounding for concrete spalls.
Is it really an unsafe structure?
Unless the person reporting the concern has a background that would allow them to know what is and is not unsafe, it is likely that there isn't an issue. A lot of concerns are based on the visual condition of a structure. This can be very misleading.
Easily visible conditions:
- flaking paint
- chipped concrete
- rotted timber
Whether or not these conditions affect the safety of the structure depend on the exact location within the structure and their extent. Most structures are designed with factors of safety that mean that almost any local issue will not cause the entire structure to be unsafe.
Also, if the condition has already been reported in an inspection, steps may have already been taken to ensure that the structure isn't loaded in a way to make it unsafe. For roadway bridges, this may mean posting vehicle load limits. For railway bridges, it may mean having trains go slower while crossing the bridge.