A ‘vulnerable person’ on the railway has caused delays between Gatwick Airport and East Croydon.
All the lines between the stations were blocked but have now been cleared, according to Southern Rail.
The rail firm said on Twitter: “The person is safe and trains are going to be able to run through the area at normal speed again very shortly.
“Some residual delays will remain to service whilst this happens.”
A spokesman for Southern added: “A trespass incident on the railway earlier today between Gatwick Airport and East Croydon is causing disruption between these stations.
“Trains are now beginning to return to normal, however, may still be cancelled or delayed whilst service recovers.”
Disruption is expected until 13:00, according to Southern.
Ticket acceptance is in place on Southern between Brighton and London where operating and on other routes to/from London, Thameslink between Brighton and London where operating, South Western Railway between Portsmouth/Southampton/Havant and London Waterloo, Southeastern between London and Tonbridge/Hastings and Great Western Railway between Gatwick/Redhill and Dorking Deepdene and change for Southern services from Dorking (a 4 minute walk) to London Victoria.
Ticket acceptance is also in place for London Trams East Croydon to Wimbledon, London Underground and London Buses to cross between London Terminals, London Buses between Redhill and London Terminals.
Tickets will also be accepted on some Metrobus services.
These are the 272 for Crawley, Three Bridges, Haywards Heath, Wivelsfield, Burgess Hill, Brighton, 271 / 273 for Crawley, Haywards Heath, Wivelsfield, Burgess Hill, Hassocks, Brighton, the 430 for Reigate, Redhill and Merstham, the 460 Reigate, Redhill, Salfords, Horley, Gatwick, Crawley and the 100 Three Bridges, Gatwick, Horley, Salfords, Earlswood, Redhill.