The 52-year-old man is believed to have forced open the emergency exit doors between carriages on a late-night Metropolitan Line Tube train between Chorleywood, Herts, and Chesham, Bucks.
Police said that he fell through a gap and, after tumbling onto the track, he was dragged along at 50mph, gruesomely severing his head from his body.
The discovery of his headless body was made by emergency service staff responding to reports that a man had fallen between the carriages.
At 1.30am the driver reported that he had been alerted that a passenger had fallen from one of the carriages and had been dragged under the train.
The power was immediately switched off on both tracks of the service to allow emergency services to attend the scene near Chalfont Latimer Station in Buckinghamshire.
It was believed his death was a bizarre accident and was not being treated as suspicious.
A rail worker who was on the Tube train at the time of the death, said on an online forum: 'A passenger who was smoking between the carriages on the train slipped and fell onto the tracks while the train was moving at 50mph.
'He was subsequently dragged to Chalfont and Latimer and decapitated.
'This just shows how dangerous messing around between the carriages really is.
'If you ever think it might be a bit of fun, or if you need a cigarette, please think again.
'Had this guy waited 10 more minutes he'd still have been with us now and nobody would have had to suffer the awful consequences of his stupidity.'
The man is due to be formally identified by his family at the hospital mortuary. Source