British Transport Police officers were called to the line in Chichester shortly after 7am ‘following reports of a casualty on the tracks’.
A spokesperson said: “Paramedics also attended however sadly a person has been pronounced dead at the scene.
“Officers are now working to identify the person and inform their family.
“The incident is not being treated as suspicious and a file will be prepared for the coroner.”
According to Southern Railway, disruption is expected until 2pm.
A spokesperson said: “We confirm that, tragically, a person was struck by a train between Havant and Barnham at around 7.15 this morning.
“Unfortunately, because the police needed to investigate the scene, we were unable to evacuate the 12 passengers on board until shortly before 9am, after a risk assessment had been undertaken in consultation with police and Network Rail to ensure the passengers’ safety.
“Supplies of water were taken to the passengers in the meantime.
“We arranged taxis to pick up the passengers at the nearest road access.
“Replacement bus services are in place between Havant, Chichester and Bognor Regis. Disruption is expected to continue until around 2pm, and we thank passengers for their patience.”
Southern said trains and stations may be ‘busier than usual’ during this disruption, which ‘could make social distancing difficult’.
“You may prefer to delay your journey until normal service has resumed,” a spokesperson continued.
“No trains can move between Barnham and Chichester. Trains that were initially held at stations between Havant and Chichester can now move forward but will terminate at Chichester.
“Leave plenty of extra time for your journey today. Trains that run on the following Southern routes are being affected by this incident.”
Whilst the emergency services are on site, the power to the track needs to be switched off in the entire Chichester and Barnham area.
Southampton Central and Portsmouth Harbour to Brighton or London Victoria trains are only running between Bognor Regis and Brighton / London Victoria.
These trains cannot run beyond Chichester so are terminating at Bognor Regis instead. You can use your Southern rail ticket on the Stagecoach bus route number 700 that runs from Bognor Regis to Chichester then onwards to Havant. Rail replacement buses are also running between Chichester and Havant, and Chichester and Bognor Regis.
From Havant your Southern ticket can be used on South Western Railway trains to Fratton and Portsmouth, you will need to change at Fratton for trains towards Southampton.
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