Services have been disrupted following a broken down train further up the line.
All Avanti West Coast services to and from London Euston are experiencing delays.
Lines have reopened but disruption between Watford Junction and Milton Keynes Central is expected until around 7.15pm.
Several services are running late including trains to Milton Keynes Central and Manchester Piccadilly.
Trains arriving from Glasgow Central, Manchester Piccadilly, Holyhead and Edinburgh have also seen delays.
Major disruption between Swindon and Didcot Parkway expected until 9pm which will affect people travelling to and from Paddington.
The emergency services are dealing with an incident between Swindon and Didcot Parkway.
Whilst they carry out their work all lines between these stations will be closed, National Rail said.
Trains may be cancelled, delayed by up to 60 minutes or revised.
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