Commuters travelling to and from London Kings Cross are facing severe disruption after signalling failed twice outside the station.
Lines were shut when a major fault was followed by another after the signalling had first been restored.
National Rail said technicians were “confident the issue is resolved” and all lines were open again but delays and cancellations would continue.
Two rail companies have warned passengers to avoid travel if possible.
Great Northern and Thameslink said they were still advising customers “to delay travelling until later”, having earlier warned against making trips during the morning “unless it is absolutely necessary”.
Grand Central, Hull Trains and LNER services are also being affected by the problem.
Passengers have been advised to expect cancellations and delays of up to 60 minutes, with disruption expected until about 17:00 BST.
Tickets are being accepted on other routes and some local bus services.