Commuters faced delays after a man allegedly forced open the door of a train at Vauxhall station and tried to walk along the tracks to Waterloo.
Passengers told of their shock after the 27-year-old man, from Basingstoke, alighted during an unscheduled stop at Vauxhall as the train from Weymouth waited to pull into Waterloo.
A passenger, who gave his name only as Sadiq, said: "People on the train were worried, some were scared something happened to the guy on the tracks."
The man was on the tracks for about 15 minutes before being apprehended by police around 8.45pm and arrested on suspicion of trespass.
Shocking footage shows trespassers lying down on train tracks
A spokesman for Network Rail said: “Trespassing on the railway is reckless, stupid and potentially lethal anywhere in the country, but in the lines to and from Waterloo, where trains are powered by a ‘third rail’ the risks are even higher.”
“Thankfully, on this occasion, nobody was injured. However, thousands of passengers would have experienced unnecessary delays to their journeys on Friday evening as a result of this person’s actions.”
A spokesman for British Transport Police said: "Police were called around 8.30pm to reports of a man walking between Vauxhall and Waterloo on the tracks.
"He was arrested on suspicion of trespassing and later released under investigation. Enquiries are continuing."