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."
– Phil Verster, route managing director for Network Rail
The Government's confirmed today that the East Coast main line, which runs through Peterborough and Stevenage, will be put out to tender after four years in public ownership. Not everyone's happy about it. Watch Elodie Harper's report.
A transport union boss has described the Government's announcement to privatise the East Coast main line as "profiteering".
General secretary of the RMT Bob Crow said: "Despite wasting hundreds of millions of pounds of taxpayers' money on the franchising circus, and instead of learning the lessons of the privatisation disasters on the East and West Coast main lines and across the rest of the network,
the Government has this morning given the green light to a whole new wave of profiteering that will have the train companies laughing all the way to the bank."
The Transport Secretary Patrick McLoughlin has said privatising the East Coast main line would put passengers "in the driving seat".
Mr McLoughlin said: "This programme is a major step in delivering tangible improvements to services, providing long-term certainty to the market and supporting our huge programme of rail investment.
"Above all, in future franchise competitions we are placing passengers in the driving seat by ensuring that their views and satisfaction levels are taken into account when deciding which companies run our railway services."