In reply to Stephen3005:
Find a friendly structural engineer to assess the integrity of the bridge and the age/solidity of the sandstone itself?
Then make sure to use glue-in staples, since expansion bolts will fail quickly on sandstone. Ideally, also put in an extra bolt low down, somewhere away from the routes, and get it pull-tested?
And if this is your first time bolting anything, get someone to help who has some experience.