A ‘safety inspection of track’ between Barnham and Chichester is causing disruption on the line.
Network Rail has said trains may be delayed by up to 20 minutes or cancelled until approximately 5.30pm.
Services are having to run at reduced speed following reports of an object on the line.
On its website, Network Rail said the object in question has been confirmed as a ‘shoe gear’, the parts of a train which run along the third rail in order to give the train power.
Network Rail are now inspecting the track and investigating why this issue would have occurred.