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

The 23-year-old was arrested after he climbed onto the rear of a train before it left Leederville Station and reached speeds of up to 110 kilometres an hour.

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

"He was clinging on to a windscreen 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 spokesman David Hynes said.

A Main Roads employee spotted the man around 4:30pm 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.

Public Transport Authority spokesman David Hynes said they had never caught someone in the act of train surfing before. ( ABC News: James Carmody )

Mr 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.

"This is the first time in the memory of anyone at the PTA that we've actually known about it while it was happening."

The PTA do not currently have a policy to deal with train surfers caught in the act but Mr Hynes said it was something they would consider.

Transport Minister Rita Saffioti said there will be an investigation into the incident and the decision to continue on to the station will be looked at during the process.

"The PTA made a judgement call and there will be an investigation, but these types of situations cannot happen and should not happen on our trains," Ms Safiotti said.

"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.