Can't build railway tracks on a bridge.

I had this issue in sunny isles. I did delete the rails, didn't work. Then i uninstalled and reinstalled the game and eventually worked - i got them back. I hope it works for you too

I can't remove the rails on both sides. I tried deleting all the rails in this region, but there was no change.

Try removing all the rails at each end of the bridge. Then leave the region alone, move to another city, close the game entirely ... see if it restores after a time.