London Victoria was left "at a standstill" because of a "major signal failure" during rush-hour.
Part of the station, the country's second busiest, was closed due to overcrowding fears. Services faced delays and cancellations until the end of Wednesday.
Southern Rail, which operates many of the services, advised passengers not to travel from Victoria.
About 75 million passengers passed through the station last year.
Images posted on social media showed hundreds of passengers held on the station concourse, unable to catch Southern, Southeastern and Gatwick Express trains.
Thameslink services out of London Bridge were also affected by the problems.
Trains were running in the area at 20:00 GMT on Wednesday, but disruption lasted until the end of service.
We are so sorry for the disruption this evening. Trains are running but far fewer than normal and we know how difficult this has been for you. A power failure to our signalling meant we lost the ability to run trains in this red square earlier and this all stems from that. pic.twitter.com/AWEf912txp— Network Rail Kent and Sussex (@NetworkRailSE) December 18, 2019
Peter Kyle, the Labour MP for Hove and Portslade, was caught up in the disruption. He described the central London hub as being "at a standstill".
Mr Kyle, said the disruption means he may miss Christmas dinner with his staff.
He tweeted: "I'm sorry to every passenger, I know there's a lot more that needs sorting on this service, I'm fighting for that. You have been let down badly this evening."
"The woman next to me is in floods of tears as she's missing her flight from Gatwick."
Rob Broomby, a TV producer, stuck at Victoria said it was the "worst transport chaos" he had seen.
He added: "There was a lot of good humour in the bar as people settled in for a long wait, but when the platform indicators began flashing on and off it felt more like a Christmas tree with dodgy wiring."
Network Rail apologised and warned that disruption could continue into Thursday morning's rush-hour.
It said: "Some trains will be finishing the day in the 'wrong' place, so we do expect there to be some disruption tomorrow morning as operators move their stock and crew around."
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