Major disruption has been caused to services at London Euston after a trespasser forced lines to be shut for more than two hours.
The delays began at about 08:00 BST and the power on the tracks was turned off so the trespasser could be removed.
The trespasser was spotted close to track near Carpenders Park, south of Watford.
The line was reopened at about 10:25, but passengers are being warned of major delays.
Network Rail said delays would have a knock-on effect throughout the day.
It has advised passengers to check with train operators before travelling.