Trains between London Victoria and East Croydon are unable to run after a person was hit by a train.
Trains are expected to be cancelled or delayed until midday, a Southern Rail spokesman said.
All Gatwick Express services have been suspended and there is no service between the two stations in either direction.
The spokesman added: “A person has been hit by a train between London Victoria and East Croydon.
“Southern services will be diverted to London Bridge where possible, and some services will be terminating short of their destination.
“Services between East Croydon and Milton Keynes Central are suspended in both directions.
“Please check customer information screens and journey planners prior to travelling.”
Ticket acceptance is in place, with Southern and Gatwick Express tickets being accepted on Thameslink services, London Buses between Victoria, East Croydon and Sutton, London Underground via all reasonable routes, Tramlink on any reasonable route, Southeastern between Victoria and Beckenham and South Western Railway on all reasonable routes.
Emergency services are at the scene and will provide an update in due course, the spokesman said.