Steel, Wood, and Concrete Railroad Bridges

My research has raised a few questions regarding the BNSF railroad bridges that are situated parallel to a major highway in the Valley of South Texas. I am trying to date these bridges. Does anyone have any information they can direct me too that tells when these bridges were constructed? Further, I am interested in the maintenance rehabilitation procedures for these concrete and steel trusses bridges. I am trying to determine if continuous upgrades and replacement parts through time suggest that these bridges no longer are original, rather they consist of parts from different time periods. I have learned that these bridges are inspected often and constantly maintained and repaired throughout history. I am thinking that these bridges are not original, meaning they have been upgraded with new and reused parts over time. Can anyone confirm this thought? Also, when you repair or replace a bridge does the railroad salvage the quality parts to be reused to repair and maintain other bridges? Another question, I have is what is the life expectancy for the concrete and steel bridges before they need repair to be replaced? Being in south Texas I know corrosion is an issue with steel bridges. I have been informed that if a steel bridge has a section that has corrosion they inspector will recommend to the railroad that this be replaced. Am I assuming correctly that the smaller concrete bridges are precast designs and then installed along the track to minimize railway down time? At what point in time were precast concrete railroad bridges used over steel and wood bridges? How long do they last and how often do they need to be serviced or replaced. Sorry for so many questions. If you have any information or documentation that you can provide I would appreciate it. If there are some publications that you can direct me to that will answer these questions, that would be even better.

Thanks,
Brian