The incident was reported at Balcombe railway station at around 2.30pm today (October 29).
Officers responded with paramedics, however, a person was pronounced dead at the scene.
The incident is not being treated as suspicious and a file will be prepared for the coroner.
All lines between between Haywards Heath and Redhill are closed with trains delayed by up to 60 minutes or revised.
Major disruption is expected until 8pm.
Southern and Thameslink services are unable to run in either direction between Haywards Heath and Gatwick Airport until further notice.
Gatwick Express services will run between Gatwick Airport and London Victoria only and Southern services that run between Eastbourne and London Victoria will be cancelled.
A spokesperson for British Transport Police said: “Officers were called at around 2.30pm today (29 October) to reports of a casualty on the tracks at Balcombe railway station.
“Officers responded with paramedics, however sadly a person was pronounced dead at the scene.
“The incident is not being treated as suspicious, and a file will be prepared for the coroner.
If you’ve been affected by what happened on the network today, there is always someone to talk to.
You can contact the Samaritans on 116 123 from any phone for a confidential chat.