Can't build railway tracks on a bridge.

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.

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

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