A person has been struck by a train between Alexandra Palace and Welwyn Garden City and all lines have been blocked.
Disruption between London and Stevenage via Welwyn Garden City is expected until 9.45pm.
Services may be cancelled, delayed by up to an hour or revised.
Great Northern Rail have tweeted: “Due to a person being hit by a train between Alexandra Palace and Welwyn Garden City all lines are blocked.
“More information to follow.”
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