The southbound rail line between Three Bridges and Horsham has been closed after a person was taken ill on a train earlier this morning (September 21).
Southern Rail said the 8.30am London Bridge service stopped on the line at Crawley.
The line has been closed as paramedics treat the patient.
Services between the two stations are being cancelled, diverted or delayed by to 15 minutes.
Disruption is expected to last until 11am.
Southern said Metrobus are accepting Southern tickets between Three Bridges and Horsham on route 23.