Potters Bar Station. (Image: Archant)
Potters Bar trains could be delayed or cancelled until the end of the day after overhead electrical wires were damaged.
Services between Potters Bar and Alexandra Palace could be cancelled, or delayed by up to 10 minutes because of the fault.
National Rail staff are working to fix the damaged wires, which are between Potters Bar and New Barnet.
Once they do, Great Northern have said they will try to restore normal service, and in the meantime, the operator has apologised for the disruption.
Passengers can follow any updates using the Twitter hashtag #HadleyWood.
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