The incident took place between Beckenham Junction and Shortlands railway stations on Wednesday.

Footage taken on board the train of the attack and 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.

British Transport Police (BTP) attended alongside London Ambulance Service and Metropolitan Police officers following reports of two men fighting.

The victim, who is in his 20s, was taken to hospital and is in a "critical but stable condition", BTP said.

The force added a knife has also been recovered.

Superintendent Darren Malpas said: "We understand the concerns of the public following this incident and our detectives have been working tirelessly to investigate.

"I hope this arrest will bring some reassurance to those on the rail network.

"High-visibility patrols will continue in the area and across the network as a visible reassurance to the travelling public."

He added: "Any witnesses who are yet to get in touch are urged to do so."