Rail services to and from London Euston are resuming after they were suspended earlier causing major disruption.
Trains were halted while emergency services dealt with an incident between Watford Junction and Bushey.
British Transport Police said a person had died at the scene at about 06:00 BST.
Lines have reopened but disruption is expected to continue until 10:00 BST.
Tickets on Avanti West Coast and London Northwestern can be used on any other operators and road transport is also being provided in some areas.
London Overground services are running as normal.
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