Commuters faced long queues on station platforms after a signal fault on one of the country's busiest rail routes caused delays.
The problem with the signalling system between Alexandra Palace and New Barnet in north London caused disruption to East Coast Main Line services.
Network Rail and First Capital Connect apologised for the disruption.
"Our engineers have been out on site since early this morning and are working to fix the problem as soon as possible.
"An investigation into the root cause of the failure is also under way."
The problem could be fixed by midday, the East Coast operator said.
"A limited East Coast service is operating via Hertford, with journey times extended by up to 45 minutes.
"Network Rail, who are responsible for signalling, track and overhead power lines, expect signalling to be restored by around 12 midday; disruption to some services can then be expected until mid-afternoon."
Click here for updated train running information.
More top news
Most places will see bright or sunny spells through the day, but with a few showers around, especially during the morning.
Most places will see bright or sunny spells through Christmas Day, but with a few showers around, especially during the morning.
A seal who sparked a rescue operation when he turned up exhausted in a farmer's field has been transported to a specialist centre in Norfolk