A failure of the electricity supply between Stevenage and Hertford North is set to cause disruption to train services until 5pm.
Journeys to and from Watton-at-Stone will take longer than expected, as journey times may be extended by up to 30 minutes.
This will have a knock-on impact on the Hertford Loop line as a whole, which also calls at Cuffley.
Gova Thameslink, which runs Great Northern services, is advising customers travelling from Moorgate who require Watton-at-Stone to travel on a replacement bus service from Hertford North to Stevenage, and change there for a bus towards Watton-at-Stone.
In both directions, Moorgate to Watton-at-Stone services will terminate at Hertford North. Watton-at-Stone station will not be served by rail.
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