Commuters are facing "major disruption" - including cancellations and delays - caused by a signalling fault.
A problem on the line between Luton and Harpenden left lines in both directions in the area blocked, Network Rail said.
The fault has been fixed but the Thameslink service between Brighton and Bedford will be disrupted until at least 13:00 BST.
Services between London St Pancras and Nottingham, Leicester, Derby and Sheffield are also affected.
Roger Perkins from Thameslink said that while the problem, which started just before 02:00 BST, has been mended, disruption will continue.
"Network Rail have now got the signalling up and running again and we will soon, we hope, be able to start running our trains again, but they really are very much backed up and completely in the wrong place.
"It's such a busy service it takes a really long time to get everything back and running on the normal timetable so there will be disruption right through the morning."
National Rail has warned passengers services may be cancelled or delayed by up to 90 minutes.
Tickets are being accepted on Great Northern, South West Trains and Southern services via any reasonable route, buses between Elephant and Castle and Elstree and Borehamwood, and the London Underground by any reasonable route.