Network Rail confirmed that about 200m of cable was missing from the main London-Scotland line, between King’s Cross and Peterborough.
All lines have now reopened, however, delays are expected.
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One Scottish passenger said that his 12pm train from London King’s Cross to Edinburgh Waverley, which went ahead, became “a free for all”.
He said: “It added stops, so everyone whose trains had been cancelled piled on. By the time we got our carriage it was a no go.
"About 100 folk didn't get on - it was beyond rammed full.”
He was unable to board the train he booked, but rail staff ensured he was first on the next train to Edinburgh.
A Network Rail spokesperson said: “We’ve made repairs and trains are now running normally again. Delays are still expected as service returns to normal. Passengers are advised to check before travelling and plan ahead.”