Trains are being delayed or cancelled between Chichester and Havant due to fault between the stations.
A level crossing fault between the stations means that trains are having to run at a reduced speed.
The delays are expected to continue up to 3pm today.
National Rail issued the following advice to customers: "Journey times between Chichester and Havant will be delayed by up to 15 minutes. Services may also be cancelled or altered at short notice.
"Your ticket is valid on the following services at no extra charge: South Western Railway between London Waterloo, Portsmouth Harbour and Southampton Central; London Underground between London Waterloo, London Victoria and London Bridge; Brighton and Hove Buses between Brighton and Shoreham-by-Sea; Stagecoach Buses on route 700 between Brighton and Portsmouth Harbour; Thameslink between London and Horsham."