A signal points failure at London's Euston station caused all lines to shut, prompting major travel disruption.
The issue has been resolved after it was first reported at about 10:40 BST, according to National Rail.
The operator has said passengers can expect delays of up to 90 minutes and cancellations, while major disruption is likely to last until 15:30 BST.
A Network Rail spokesperson apologised to customers.
They said in a statement: "We're sorry for the disruption to services in and out of London Euston earlier today while our engineers repaired a fault with the signalling system in the area."
Avanti West Coast and London Northwestern Railway said tickets impacted by the disruption would be valid on some other train services.
Ticket acceptance has also been agreed with East Midlands Railway and Thameslink between Bedford and Wellingborough to assist passengers travelling to or from London St Pancras International.
BBC journalist Alex Smith, who is at the station, said: "I've spoken to some staff who say that they don't know what trains are turning up between platforms one and seven, and where they are going to - resulting in numerous, last-minute dashes for trains as they get announced.
"It is absolute chaos."
