Horrified passengers watched as a man fell in front of a train as it sped through a busy station.
Families were among those who witnessed the suspected suicide as they waited on the platform at Portslade on Saturday.
The victim, who has not yet been identified, was killed instantly as he was hit by the high-speed train from Basingstoke to Brighton shortly after 11am.
The train driver slammed on the brakes when he saw the man plunge from the platform but could not stop in time. He and other people who saw the incident were later offered counselling.
British Transport Police closed the station and turned off the power while they recovered the body.
Passengers were escorted off the train, due to arrive in Brighton at 11.12am, and directed to replacement bus services.
The line reopened at 1.15pm but signal failures meant services were disrupted for the rest of the day.
A British Transport Police spokesman said the death was not believed to be suspicious.
He said: "We have spoken to the train driver who was obviously the prime witness. It was a large train and even travelling at a slow speed would have been very difficult to stop."
Glenn Murphy, 24, who works at The Whistlestop Inn in Station Road, Portslade, said: "At first we heard someone had been struck by lightning.
"Everyone knew about it because the station was cordoned off with police tape and you weren't allowed to go in."
One woman said she heard shouts and screams moments after the train hit the man.
She said: "There were lots of mums with buggies sitting on benches on the platform. I didn't see anything but after the train had gone past there was shouting and screaming.
"Then, after a few seconds of stunned silence, quite a few of the mums packed up their buggies and left in a hurry."
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