A woman leapt in front of a train in an apparent suicide attempt.
She jumped off the platform at Preston Park station at 12.25pm into the path of the 10.10am Bedford to Brighton train.
The woman, who has not been named, was taken to the Royal Sussex Hospital by air ambulance.
A spokeswoman for British Transport Police said she was still alive but is likely to lose both legs.
She said: "It is being treated as non-suspicious at the moment."
Trains in the area have been severely delayed while investigations take place.
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