On a mountain bike, that is, with a long video at the bottom, replete with spagetti western music. Most of the route is a legal ride, but the tunnels and trestles are now trespassing, with recent efforts to reopen the line. Wouldn't want to be without a good working cell phone, as the tunnels look to be the safest part of the ride, mostly a narrow, rocky path with very steep drops.

http://www.mountainbikebill.com/CarrizoGorge.htm