There are delays of up to 20 minutes between Raynes Park and Epsom after a train broke down at Worcester Park station.
The 11:09 London Waterloo to Guildford service, via Epsom, had "developed a fault" and was delayed at Worcester Park for 50 minutes, according to a South West Trains spokesman.
Disruption on lines from London Waterloo to Dorking, Epsom and Chessington is expected until 1.30pm.
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