Thousands of passengers in London have been hit by rail chaos after all trains to and from Euston were cancelled and a last-minute strike in France suspended Eurostar services at St Pancras.
A faulty train led to the disruption of Avanti West Coast trains between Euston and Scotland.
Euston was closed to incoming passengers and operating an intermittent "exit only" system.
Network Rail said lines had reopened but delays would last until 19:00 GMT.
Services between London Euston, Birmingham, Liverpool, Manchester and Glasgow were affected.
Initially, it was thought the problem was damaged overhead power cables but Network Rail said an investigation had found the issue was a faulty train in the Bourne End area at 12:15 BST.
"All lines were closed while investigations took place into the incident, including drone surveys of the lines, but trains were on the move soon after 15:00," Network Rail said.
Trains forced to return
Dozens of Eurostar trains have also been cancelled due to what the company called a "last-minute" strike by French Eurotunnel workers in a row over pay.
Several trains were already heading towards the Channel Tunnel when they were forced to return to their starting point. Eurostar says that no more trains will run on Thursday.
Thirty of its trains were cancelled out of London, Paris or Brussels, Eurostar said.
The company says that passengers due to travel tomorrow will be updated with any changes to services from around 19:00 GMT.
Customers who were affected on Thursday "have the option to exchange their tickets free of charge or claim a refund", Eurostar says.
"Although the situation is out of Eurostar's control, all our teams are working to minimise the impact on our customers and provide as much support as possible. Extra staff are also in the stations to assist travellers," it adds.
Meanwhile services in and out of King's Cross are also subject to delays or cancellations because of a tree falling on the East Coast Main Line near Newark.
Passengers at King's Cross posted videos of the station's concourse, which was full of people and watching departure boards with numerous cancellations showing.
Strong winds from Storm Pia have caused much of the weather-related disruption.
At St Pancras International
Simon Jones, BBC News correspondent
There are big crowds - and lots of glum faces. People have no idea how they are going to get home across the Channel for Christmas.
A tannoy announcement has informed passengers that even if Eurostar services do resume later today, they are fully booked.
Some people have been scrabbling around to book hotels; others have been trying to find flights or even contemplating getting on ferries from Dover.
The advice for passengers, as ever, is to check before they travel - but the problem is Eurostar and Eurotunnel have very little information to pass on.
So for many, it's a waiting game.
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