British Transport Police have condemned a video which shows a person standing on the outside of a train travelling on Liverpool's northern line.
The video - which was posted online - shows someone standing on the coupler between two carriages while the Merseyrail train travels at speed in an act of so-called "train surfing."
The footage, which officers have described as 'extremely concerning', has since been deleted from social media.
British Transport Police have a copy of the video and are continuing investigations to identify the culprits.
PC Jane McGee said: “Although relatively rare, this is incredibly dangerous and remarkably stupid.
“We are increasing patrols in the area we suspect this happened and appeal to those tempted to put their lives on the line like this to think again and consider just how dangerous it is and the likely consequences. They are gambling with their lives.”
Greg Suligowski, Head of Communications at Merseyrail, said: “It is extremely concerning to see this video which appears to show an individual trespassing on the railway, placing themselves and others at significant risk.
“We are working closely with the British Transport Police to identify those involved so that appropriate action can be taken.
“The non-public areas of the railway can be a very dangerous environment and I would urge people not to attempt to access them under any circumstances.”
Anyone with information is asked to contact British Transport Police on 0800 40 50 40 or text 61016.
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