Police said that the man stabbed during a fight on a train to Beckenham Junction is in a critical but stable condition.
A 19-year-old has been arrested suspicion of attempted murder after the stabbing after officers were called to the station at just before 4pm on Wednesday.
Units from the British Transport Police, London Ambulance Service, and Metropolitan Police responded.
A man in his twenties was taken to hospital, where he remains in a critical but stable condition.
The knife has been recovered.
Station high-visibility patrols are ongoing.
Assistant Chief Constable Allan Gregory said: “This is an incredibly concerning incident which took place in front of other passengers in broad daylight.
“Overnight and into Thursday, there will be an enhanced police presence at stations in the area as officers continue to carry out extensive enquiries.
“We do not believe that there is any risk to the wider public."
BTP are appealing to anyone who witnessed the incident to contact BTP by texting 61016 or by calling 0800 40 50 40, quoting reference 397 of 27/03/2024.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.
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