All trains were halted to and from Euston station following a fatality on the tracks.
Lines were blocked as emergency services responded to the scene in Watford, meaning mainline trains could not run, but they have since reopened.
In its latest update, National Rail said “major” disruption is expected between Watford Junction and Euston until at least 7pm.
They added: "Whilst service recovers, trains running between these stations may still be cancelled, delayed by up to 60 minutes or revised."
The incident has been affecting passengers travelling with Avanti West Coast, London Northwestern Railway and Southern.
Avanti West Coast between Blackpool North / Edinburgh / Liverpool Lime Street / Manchester Piccadilly and London Euston is affected.
London Northwestern Railway between Birmingham New Street / Coventry / Bletchley / Milton Keynes Central and London Euston is also affected.
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