We were called at around 5.30pm following a report a man in his 20s had received a serious head injury while travelling on a train between Balham and Wandsworth.
Emergency services rushed to the scene but despite their best efforts, nothing could be done to save him.
Our investigation remains at an early stage, but initial enquiries suggest the man may have been leaning out of a train door window when he suffered a blow to the head.
Officers are currently working to confirm his identity and inform his next of kin.
Inspector James Tyrrell from BTP said: “We are aware of a number of reports on social media which say the man was decapitated, however this is not the case.
“We are continuing to investigate the circumstances of the incident, which has been reported to the Rail Accident Investigation Branch (RAIB), and I’d urge anyone who saw what happened to contact us.”
The death is not being treated as suspicious and a file will be prepared for the coroner.
Anyone with information is urged to call BTP on 0800 40 50 40 ref 371 of 07/08/16.