How can I mount a railing in concrete steps when I encounter rebar in the hole?
New bit and a hammer drill will get you through it. Spent my 1st summer between college installing iron railing and that's the way we did it for railing that had tabs on the bottom and there was nothing we couldn't get through. If you hit rebar tip the drill at an angle and go on the side of the rebar. 

For non tab installs we core drilled and used hydraulic cement.