Trains are delayed between Surbiton and Woking after a member of staff became ill.
Disruption on the line is expected until 7pm, according to South West Trains.
A spokesman for the service said there had been a "medical emergency" at West Byfleet, and the 16:20 service from Waterloo to Exeter St Davids had come to a stand just outside the station.
The spokesman said: "This means trains may have to be cancelled, altered or revised at short notice.
"Some services may also be diverted via an alternative route to avoid the affected area. Further information will be provided when known.
"We are sorry for any disruption to your journey this evening."
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