The East Coast service to Edinburgh left London King's Cross at 11am on Monday with hundreds of passengers on board, and finally arrived at 11.41pm -seven hours and 53 minutes behind schedule.
The company had to cancel almost 90 services, as well as disruptions to other services, affecting more than 30,000 passengers, after overhead wires, which are the responsibility of Network Rail, came down on to the tracks north of Peterborough.
The Edinburgh train was passing the area when the power lines failed. A rescue locomotive was attached to the train and the service resumed its journey, but this broke down at Dunbar.
East Coast said passengers had been contacted and were being urged to claim compensation.
East Coast managing director Karen Boswell said: "This incident has been extremely frustrating for our customers, and I want to say sorry to everyone whose journey was delayed.
"This is the latest in a series of incidents related to the infrastructure on the East Coast line, and currently, overhead line failures are the biggest cause of delays."
Phil Verster, Network Rail route managing director, London and North East, said: "We can only apologise for the incident. This failure mode in the overhead lines appears to be an inherent design issue that was not detectable. Our engineering teams are working as quickly as possible."
Some services were delayed yesterday as a result of the incident.