A man was seriously injured when he was attacked by three passengers on a train in south-east London.
It is believed the victim, aged 61, was knocked unconscious when he was pushed from the train as it stopped at West Dulwich station at 2140 GMT on Monday.
A doctor treated him at the scene before emergency crews took him to King's College Hospital - he has since been transferred to another hospital.
A 19-year-old man from Brixton has been arrested by police.
The victim was travelling on a service to Orpington which left London Victoria at 2122 GMT when he was involved in a struggle with three other passengers - two men and a woman.
A passenger on board at the time of the incident told BBC News: "She [the woman] was doing the usual tout for spare change but there was something not quite right - they were checking the passengers out too closely.
"One of her henchmen put up his hood and sat behind me - that's when I decided to exit the train at the next opportunity.
"His mate turned back and went back into the carriage he had just come from. As I got off the train at West Dulwich this poor fellow came flying out of the exit next to me and hit the platform - he was really badly injured."
The passenger added: "It all happened so quickly. The attacker got away but passengers stopped the other man from leaving and police arrested him when they arrived.
"We don't know where the girl went. It was horrible, saddening, shocking."
Officers closed the platform after the incident and took the train out of service, forcing other passengers to use local bus services.