Signal problems are being blamed for delays and cancellations between Three Bridges and Horsham.
Rail operator Southern says trains using the route may be cancelled or delayed by up to half and hour.
A statement from the company said: “Disruption is expected until 17:45 23/05.
“Train services via Horsham are having to run at a reduced speed into platforms 3 and 4 due to the signalling fault at Horsham.
“Although, the fault is via platforms 3 and 4 this may also lead to delays through platforms 1 and 2 as trains are having to come to a stand to gain permission from the signalling centre to enter the station.
“Network Rail who own and maintain the track and signalling system are on site and we will update you with the situation as soon as we are advised.
“We are in constant discussion with Network Rail in order to keep any delays, alterations and cancellation to a minimum.”
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