British Transport Police, Sussex Police and paramedics attended the scene at around 6.30pm, following reports a person had been hit.
Sadly, the person was pronounced dead at the scene.
A spokesperson for BTP said: “The incident is currently being treated as non suspicious. Medics from South East Coast Ambulance Service also attended but the person was pronounced dead at the scene.
“A file will be prepared for the coroner.”
Southern Rail confirmed the train involved in the incident was 17.29 service from Portsmouth to Brighton.
Rail services were delayed by around an hour as the incident was dealt with.
A spokesman said the line was set to re-open shortly.