Delays are being reported on the railway lines between Horsham and Three Bridges after a woman was spotted sitting next to the tracks.
British Transport Police said officers were called to the incident near Ifield after receiving a report from a member of the public of a woman sitting next to the tracks.
A spokesman said police searched the area but could not trace the woman.
Southern Rail said the incident took place near the Ifield level crossing at around 4.15pm.
Trains are running at reduced speeds between the two stations.
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