A video has emerged on social media of a man leaping off a moving train on the Fremantle rail bridge into the Swan River, prompting an investigation by transport officials.

Authorities have confirmed the dangerous train surfing stunt took place on Saturday morning.

The man climbed on the top of a train at North Fremantle Station at about 10:30am before jumping from the carriage as it crossed the bridge, narrowly missing live overhead wires as he leapt into the air.

He plunged into the water just metres from a concrete pylon.

The Public Transport Authority (PTA) said it was working with police to try to identify the man from CCTV footage.

He could be charged with trespassing, an offence that carries a penalty of up to 12 months in jail.

Stunt 'gobsmacking', transport authority says

PTA spokeswoman Claire Krol described the man as an idiot.

"It's absolutely gobsmacking," she said.

"I first thought this had to be a joke, someone has edited [the video footage], but it turns out it's actually real.

"I can't believe anyone would put themselves on a moving train less than a metre below 25,000 volt wires. He is so lucky to have survived."

Ms Krol said the man was not the only one who could have suffered the consequences.

"If this had gone wrong there are people like water police, ambulance officers, train drivers and transit officers who have to respond to this," she said.

"That's a really bad day at work for somebody, particularly when this guy has gone out of his way to do something that is just so stupid.

"I know why people are watching this vision because it is jaw dropping, but unfortunately it does encourage copy cats."

The stunt comes six months after a man who was filmed clinging to a windscreen wiper on the back of a moving train was fined $1,000.

The man climbed onto the rear of a train before it left Leederville Station and was recorded by a motorist on the Mitchell Freeway as the train headed north to Glendalough station.