Teenagers have been caught on camera "surfing" on the back of trains in Australia's biggest city. Footage online showed one 17-year-old hanging on to a carriage as it hurtles across Sydney Harbour Bridge at high speeds. Video shot by the teenager on a head-mounted camera also shows him clinging on to the train with the track just a few feet below. Authorities warn some young people have died in accidents when performing the stunt and say they will enforce the strictest penalties on anyone caught.