A man was hit by a train when he fell on to the tracks at Twickenham station as thousands of rugby fans were making their way home from the opening night of the World Cup.
The victim suffered head and leg injuries and is being treated in West Middlesex Hospital.
British Transport Police (BTP) are investigating how the man came to fall on to the tracks at platform 3 but said early indications suggest it was an accident.
The station was temporarily closed as emergency services dealt with the incident at around 11pm, and services were delayed for a further two-and-a-half hours when it reopened.
A spokeswoman for BTP said: "The man was struck by the train (10.33pm Windsor to Waterloo service), which was slowing down to come into the station, and suffered head and leg injuries as a result of the incident."
She added: "At this stage it is unclear exactly how the man came to fall onto the tracks, though early indications suggest it was an accident, and officers are now working to determine the full circumstances."
Some of those attempting to get home said the situation was "chaos".
Around 82,000 people including Prince Harry and the Duke and Duchess of Cambridge attended the first game of the tournament on Friday at Twickenham stadium, to see hosts England beat Fiji.
A spokeswoman for the South West Trains - Network Rail Alliance apologised for the disruption passengers faced, adding: "Our immediate thoughts are for the welfare of the person involved."
Pictures posted on social media showed large crowds at both Twickenham and neighbouring Richmond stations as people attempted to leave the area.
Jack Fryer tweeted: "Completely stranded at Twickenham. Travel chaos. Taking refuge in hostelry. Medieval stuff."
Another man described the scenes at the station as "ridiculous", saying that he walked two miles to Richmond station as an alternative to get back to east London.
Mark Goodrich called for improvements at Twickenham station, which is expected to see huge crowds during the course of the tournament.
He said: "Good to hear that tonight's incident at Twickenham station after RWC2015 not fatal. Infrastructure badly needs overhaul."