Trains to St Albans and Radlett could be delayed this evening.
A safety inspection was carried out on the line between Mill Hill Broadway and Radlett this afternoon.
As a result, trains to Radlett and the Thameslink service to St Albans could be delayed by up to 15 minutes.
Services going towards St Pancras International are also having to run at a reduced speed.
The disruption is expected to clear by 6.30pm.
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