Cuffley passengers face delays of nearly an hour due to train signalling problems at Hertford North.
Govia-operated Great Northern says services to and from Hertford North may be cancelled at short notice.
There will also be a reduced service from Cuffley, Crews Hill, Hertford North and Bayford.
The fault lies with multiple signals being displayed as red, meaning danger, which means train drivers need to verbally authorise their progress along the route with the signaller.
This has resulted in delays with trains moving.
Watton-at-Stone is not served at this time. Passengers at the station, should board the rail replacement buses to Stevenage. From there, you can change for mainline services towards London or Bus services to Hertford North.
Disruption is expected till 4pm.
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