All Thameslink rail services between Gatwick and Three Bridges in Crawley have been suspended after a person was hit by a train.
The operator said emergency services were at the scene and major disruption was expected until 09:00 GMT.
Travellers are being asked to find alternative routes, with journeys likely to take at least an hour longer than usual, with other operators accepting Thameslink tickets.
It is not yet known if the victim has died.