A man has died after leaning out of a train window and being hit by another train, it is understood.
The Gatwick Express passenger was pronounced dead at the scene after the incident at Wandsworth Common station.
A spokeswoman for London Ambulance Service said: "We were called at 5.34pm to reports of an incident at Wandsworth Common railway station.
"We sent an ambulance crew, a single responder and a car and an incident response officer to the scene alongside an air ambulance.
"We treated a person at the scene for a head injury but sadly they were pronounced dead by the doctor from the air ambulance."