Train travellers are facing major disruption after a length of overhead wire fell on to the line.
Problems on the lines between Radlett in Hertfordshire and St Albans last night means Thameslink services between Sutton and St Albans and Luton are subject to severe delays and cancellations.
Delays are expected to affect the lines throughout the day and into Friday.
The problems are thought to have been caused by a kilometre of overhead cables falling onto the line near Radlett just after 11am yesterday. Five trains near Radlett had to be evacuated.
Some repair work has already taken place but work will not be completed until tonight and lines the lines will not be fully open until tomorrow morning.