Thousands of passengers faced travel chaos after a man jumped in front of a train.
The man was inside the tunnel near Hassocks station as the train approached shortly after 3pm on Tuesday.
Commuters on other trains were caught in long delays after rush hour rail traffic was held back as emergency crews arrived at the scene.
Both tracks between Brighton and Hassocks, on the busy Brighton to London line, were closed for more than two hours.
Inspector Gary Young, of the British Transport Police, said the man was seen walking along the inside of the tunnel.
The driver saw him jump in front of the train, which was travelling at about 85mph.
There were no passengers on it.
Commuters faced substantial delays as they continued their journeys on buses or were redirected via Lewes.
A spokesman for Southern Rail said: "There was a lot of disruption.
"Delays were up to 60 minutes while the lines were closed but fortunately they were re-opened just after 6pm."
Mr Young said police were not treating the death as suspicious. They are still trying to identify the victim.
An inquest is to be held into the death.
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