Trains are being delayed or cancelled due to a family of swans on the railway between Chichester and Barnham
Journeys to and from Brighton, Portsmouth and Southampton were extended by up to 30 minutes, a spokesman for Southern said.
The lines reopened at 8.30am but services remain subject to delays and are expected to until about 9.30am.
A statement on Southern Railway's website read: "Train crew on board the 06:12 Portsmouth Harbour to Littlehampton service had reported a family of swans are on the line, in front of the train. As a result, this service including others within the vicinity were unable to proceed onwards.
"Despite the driver's best efforts to persuade the swans back to safety, they continued to remain on the tracks and made themselves clear that they didn't want to move.
"Network Rail response teams were called to site and were required to isolate the line with the attempt to remove the swans. Initially, this proved difficult as the swans were distressed, however, due to the best efforts from staff on site, all swans are now safely on their way home."