Commuters faced train delays after police removed a man who had trespassed on the tracks.
Thameslink confirmed that emergency services were called to Radlett station, near Watling Street, just before 5pm. Police removed and detained a trespasser by 5.17pm.
Passengers heading towards Luton Airport reported being stuck in Elstree with no indication of when they would be able to start moving again.
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The 15.19 Sutton to St Albans City train was initially delayed and then caused further delays to services behind it.
Hertfordshire Constabulary have confirmed they were called, alongside British Transport Police.
Herts officers were still in attendance as of 5.30pm, but British Transport Police are leading.
All lines have reopened.
Journeys between London St Pancras and Luton are expected to be disrupted until 6pm.
National Rail said: “An earlier trespass incident at Radlett is causing disruption to journeys between London St Pancras International and Luton.
“Services may still be cancelled, delayed by up to 25 minutes or revised.”
It warned late-running trains may stop at fewer stations en route.
British Transport Police have been contacted for more information.
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