Dozens of motorists on the Mitchell Freeway watched a man clinging to the outside of a moving train in Perth, Australia, on Saturday afternoon.

The 23-year-old was arrested after he climbed onto the rear of a train before it left a suburban station and reached speeds of up to 110 km/h.

The Public Transport Authority (PTA) said he could easily have been killed.

"He was clinging on to a windscreen [windshield] wiper — were he to fall it's a very unfriendly and unreceptive surface with ballast and rail and concrete sleepers, he would have almost certainly been killed, if not badly injured," PTA spokesperson David Hynes said.

A roads authority employee spotted the man around 4:30 p.m. local time on Saturday between Leederville and Glendalough stations while monitoring traffic, and immediately notified the PTA.

The PTA said by the time they could react, the train was close to Glendalough Station where it was scheduled to stop.

The PTA monitored the situation on CCTV as the man got off the rear of the train and went inside the carriage.

He was detained after the train arrived at the next stop, Stirling Station, by waiting PTA officers.

'People shouldn't be acting stupidly'

Hynes said it was the first time the PTA has ever been made aware of a train surfer during the act.

"In all other instances it generally goes up on Facebook or on the internet, on YouTube after the event," he said.

"They want to get on YouTube, they want mainstream publicity so they don't try to get themselves caught, so we don't often know about it until it's well after the event.

The PTA does not currently have a policy to deal with train surfers caught in the act, but Hynes said it would consider drafting one.

Transport Minister Rita Saffioti said there will be an investigation into the incident, including the decision to continue on to the station when the PTA knew there was a man clinging to the train.

"People shouldn't be acting stupidly around our rail line, these are serious pieces of infrastructure and they can cause death and of course they can cause serious injuries," she said.

The man was taken to hospital to undergo a mental health assessment and could face charges for trespassing on Public Transport Authority property.