Southern Rail said a person was struck by a train between Angmering and Brighton.
Trains running between these stations may be cancelled, delayed or revised with disruption expected until 2pm.
A statement from Southern said: ‘It is with great sadness that we report a person has been hit by a train between Worthing and Littlehampton / Barnham.
‘All lines are closed between these stations whilst the emergency services work to deal with this incident.’
Southern are expecting severe disruption and have advised people to travel later – with delays of one hour being added to journey times at present.
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The rail firm is working to arrange alternative journey options and will let travellers know when these are in place.