People are being advised to delay their journey.
A spokesman for Southern Rail said in a tweet: “It is with great sadness that we report a person has been hit by a train between Brighton and Worthing.
“All lines are blocked between these stations whilst the emergency services work to deal with this incident.
“If you’ve not yet set off for the station, you should delay travelling until later if you can.
“You can use your ticket on: Brighton & Hove buses, Stagecoach 700.”
More to follow.