A man has been arrested after being spotted by dozens of freeway motorists clinging to the windscreen wiper of a moving train in Perth, prompting an investigation.

Public Transport Authority (PTA) spokesman David Hynes said on Sunday that the man boarded the train at Leederville and was filmed by the public clinging to the exterior of the train about 4.30pm on Saturday.

Main Roads cameras initially saw the man just before the train arrived at Glendalough station and he then moved to the inside of a carriage.

The 23-year-old man was arrested as the train arrived at its next stop at Stirling.

The train reached speeds of up to 110 kilometres an hour.

A police spokesman said he was taken to Sir Charles Gairdner Hospital for a mental health assessment.

Mr Hynes of PTA said the man would face trespass charges.

“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,” he told the ABC.

Mr Hynes denied any suggestion the PTA was slow to halt the train, saying it would take 800 metres to stop and the man moved into the carriage shortly after the PTA was alerted to his actions.

He added it was the first time the PTA had 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.”

WA Transport Minister Rita Saffioti described it as a “very serious case” and said the transport authority had made a judgment and there would be an investigation into what had happened.

“These cases of pure stupidity can’t really be accepted … they’re putting that person’s life in danger and, of course, the passengers in danger,” she told reporters.

Ms Saffioti said she would wait for the investigation to be completed before passing judgment on whether the PTA acted appropriately.

-with AAP and ABC