A man has died after reportedly leaning out of a train window and being hit by another train.
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."
A spokesman for Gatwick Express, which is run by Southern railway, said it first received reports about the incident at 5.45pm.
He said: "We have received reports of an incident on board a Gatwick Express train which was en route to Victoria from Gatwick this afternoon.
"The emergency services are in attendance at Wandsworth Common station. We have no further information at this stage."
A spokesman for British Transport Police said the incident was not being treated as suspicious, and that a file is being prepared for the coroner.
Southern Rail tweeted that services between Victoria and Balham were being delayed, saying: "Emergency services dealing with an incident between Balham & Wandsworth Common."
Residents said they were shocked to see the air ambulance land on the common earlier in the afternoon.
One said: "We were coming back across the common when we saw the helicopter land.
"It was at about 6pm. There were lots of police and ambulances there, it was disconcerting."
Another said: "I heard a man was hit by a train and heard the air ambulance land on the common. It's such a horrible thing to have happened."