Harbour Bridge daredevils risk their lives for videos

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.

Clinging to the emergency door, he records the tracks shoot out underneath him as the train whips over the harbour.

The teens post videos of their death-defying stunts online.

It's all for the thrill, but authorities are not impressed.

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

"One small slip as you're going over that Harbour Bridge and you're going to be dead."

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.

Incredibly they make it safely to Milsons Point before running away.

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

"It just shows a complete lack of awareness of the risk to personal safety," he said.