Is there an app that will let me know the height of bridges or other obstacles in my area I have some railroad bridges that are extremely high and would like to know the elevation to safely fly over

Not sure where you are from but I would suggest learning to read what we call in the US Sectional Charts, used in aviation.  They provide details of all obsticales and provide height information.  You can see online versions for the US at https://skyvector.com/

Not enough detail,looking for a way to measure particular bridge or large powerlines ??

Fly up and look at the camera view to see when you've cleared the required height.