All morning trains are cancelled between St Albans Abbey Station and Watford Junction.
The disruption began at around 8.30am today (Friday, August 19).
The journey would normally also include stops at Park Street, How Wood, Bricket Wood, Garston (Hertfordshire) and Watford North.
According to London Northwestern Railway, a "local operational issue" is the cause of the disruption.
A spokesperson for London Northwestern Railway said: "Problems reported.
"Cancellations to services between St Albans Abbey and Watford Junction: Due to a local operational issue between St Albans Abbey and Watford Junction, all lines are blocked."
Services are due to resume in the afternoon.
There are strikes taking place on the Transport for London (TfL) network today (Friday, August 19) and on the National Rail network nationwide tomorrow (Saturday, August 20).
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