The train operator say that lines have now reopened but that delays are set to continue until at least 11am.
Trespassers made it onto the railway somewhere on a ten mile piece of track between Welwyn Garden City and Potters Bar, stations around 20 minutes north of London.
Great Northern tweeted: “All lines have now reopened following trespassers on the railway between Welwyn Garden City and Potters Bar.
“Services may be delayed by up to 40 minutes or subject to alteration/cancellation.”
Great Northern has urged customers to consider alternative travel arrangements and confirmed that tickets will be accepted on Thameslink services between Cambridge, Peterborough and London.
The company confirmed that delays were caused due to power being turned off which had prevented train stock from leaving the depot on time.
Emergency services were called to detain the trespassers in the early hours of the morning and the line was reopened by 06:17am.
In a Twitter statement, Hertfordshire Police said: A person has been safety taken off the tracks following this incident. Trains are now running again, but please check with your service provider as there may still be delays. Thanks for your patience and understanding.”
But commuters were left horrified by the delays which came in the heart of Monday morning rush hour.
Paula Baldwin of Palmers Green, North London said: “It’s just so selfish that the stupid actions of one person can make thousands of people late for work. This train line is already bad enough without trespassers making it worse. I hope whoever it is gets nicked and ends up in front of a magistrate this morning,”
Trespassing on the railway reached a five year high in 2018, according to statistics from Network Rail. The company launched the You Vs Train campaign last year alongside the British Transport Police to raise awareness of the dangers of stepping onto live railway tracks.
A Network Rail survey showed that 72% of trespass incidents between February 2017 and February 2018 took place within 100m of a train station, while 15% of 13 to 18-year-olds thought it was safe to walk on a railway track if there was no train in sight.