A possible defective track is causing delays to train services between Hatfield and Potters Bar.
Shortly after 8.30am Govia-run Thameslink reported a possible track defect affecting a London bound train - which a second train confirmed.
Network Rail has sent staff to investigate and - as a safety precaution - trains will run at a reduced speed across the affected section until the track has been looked at.
Some services will be using an alternative London line.
As a result, trains might be cancelled, delayed by up to 30 minutes or revised until the disruption ends at around midday.
For more information, go to greatnorthernrail.com/travel-information/plan-your-journey/service-updates.
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