Young people are risking their lives to train surf across one of Sydney's most famous landmarks, in a dangerous new trend.

Video footage taken with a Go Pro camera shows a 17-year-old hanging off the back of a train as it flies across the Harbour Bridge.





















"It's just pure idiocy," Sydney Trains' Howard Collins said.





In another incident, right in the middle of school holidays, a pair was also caught on camera train surfing the bridge.

They jump the turnstiles at Wynyard and loiter near the back of the train as one straps a Go Pro to his head and they leap aboard.





Acting Superintendent David McBeath of the Police Transport Command said the youngsters were taking an incredible risk with their lives.