An electricity supply failure is causing disruption on the St Albans, Stevenage, Welwyn Garden City, and Hatfield train lines this morning.
A short circuit at Three Bridges station means that no services can currently run between Three Bridges and Horsham in either direction.
The blockage is causing delays and cancellations for railway users along the Bedford to Brighton Thameslink line and the Peterborough and London Great Northern line.
The problem was first discovered on Three Bridges platforms one and two this morning, with Network Rail insolating and replacing a defective insulator pot.
After further issues stabilising the power supply, engineers found an object had become wedged beneath a third rail.
Assessments of the line are ongoing and Network Rail estimate that services will be disrupted until 12pm.
Use #ThreeBridges to follow the incident on Twitter.
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