British Transport Police have also recovered a knife.
A 19-year-old man has been arrested on suspicion of attempted murder after a stabbing on a train in south London.
British Transport Police (BTP) said they received reports of two men fighting between Beckenham and Shortlands railway station shortly before 4pm on Wednesday.
Footage shared on social media showed a male wearing a black hooded jacket, black trousers and a face mask holding a large blade, while onlookers could be heard telling him to stop.
A manhunt was launched and on Thursday BTP said they had arrested a 19-year-old man and recovered a knife.
The victim, who is in his 20s, was taken to hospital and is in a “critical but stable condition”.
