Railway lines have been blocked between Horsham, Littlehamtpon and Barnham after a person was struck by a train.
Police said officers were called to Arundel station at 2.15pm following reports a person had been hit by a train at a level crossing at Offham north.
Southern Rail said services between Horsham and Barnham had been cancelled or delayed by up to 45 minutes following the incident.
Southern said it was looking to arrange replacement buses to run between the stations but was advising people to leave extra time for their journeys.
Disruption is expected to last until 5pm.