A points failure at Harpenden station is causing disruption to trains running through the station between London St Pancras and Luton.
Trains on this line may be cancelled or delayed by up to 15 minutes.
Disruption is expected to continue until 9am.
Tickets will be accepted on East Midlands Train services between St Pancras and Bedford for Luton Airport Parkway.
Network Rail engineers are on their way to the site.
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