A teenager is believed to have been shot with a crossbow bolt - in what police have called a "shocking act of violence".
The 18-year-old victim was found with serious injuries on a bench at Abbey Road station in southeast London around 10.20pm on Sunday night.
According to British Transport Police, he was taken to hospital and his condition is described as life-threatening. No arrests have yet been made.
The station reopened on Monday morning after being shut while officers investigated the scene.
Detective Superintendent Sam Blackburn said: "We are at an early stage in our investigation and we are keeping an open mind as to exactly where this violent incident began.
"Following a number of inquiries throughout the night, we believe the man sustained his injury from a crossbow bolt.
"I would like to appeal to anyone who may have witnessed anything suspicious along Abbey Road and Wilton Road between 9.45pm and 10.20pm to get in touch as soon as possible to assist the investigation."
The detective added: "This was a shocking act of violence and I'd like to reassure the public we're doing all we can to find those responsible."
British Transport Police are urging anyone with information to text BTP on 6101 or call 0800 405040 quoting reference 341 of 14/02/21. Alternatively, please call Crimestoppers anonymously 0800555111.