Rail regulator to investigate rail work delay chaos
The Office of Rail Regulation is to launch an investigation into major disruption to passengers caused by overrunning engineering work in London.
King's Cross station has remained closed, with travellers told to use Finsbury Park instead.
Following "chaotic scenes" at Finsbury Park, passengers faced long queues to get on the platforms for a time.
Network Rail managing director Robin Gisby apologised, saying: "We've let a lot of people down today."
Transport Secretary Patrick McLoughlin is demanding an explanation from Network Rail, which is responsible for the UK's train network.
Engineering work delays also mean there are no services between London Paddington and Reading.
The work between King's Cross and Finsbury Park is part of a £200m Christmas investment programme, Network Rail said.
Mr Gisby told BBC News: "We've let a lot of people down today I'm afraid, and I can only apologise for that.
"I've been out and about and have witnessed just the impact that our engineering work has had on passengers at Paddington and King's Cross."
He said Network Rail had run about 10,000 trains and had tried to run a "reasonable service" on top of the "necessary" engineering works.
"But unfortunately some signalling problems at Paddington have gone badly wrong until early this afternoon, and we had some machinery break down on Christmas Day into Boxing Day, to the north of Kings Cross," he said.
A decision was taken on the afternoon of 26 December to put alternative arrangements in place and make an announcement to passengers, he said.
"I'd much rather do that and make the call, than hope it all came right," he said.
He added Network Rail was doing all it could "not to repeat the dreadful disruption caused today".
Previous investigations by the Office of Rail Regulation have led to multi-million pound fines.
A spokesman for the regulator said: "The immediate priority is for Network Rail to ensure disrupted parts of the railways are back up and running again for passengers as soon as possible.
"Network Rail, working with the rest of the industry, must learn lessons and prevent problems like this happening again."
'A real person telling us would be nice'
By Emily Unia, BBC News
The queues at Finsbury Park snaked from the station entrance, along the pavement and round the corner.
People with small children, large suitcases, dogs and bicycles all waited in the cold. A common complaint was the lack of information about what was happening.
A woman trying to travel to Edinburgh said "not all of us are on Twitter - a real person telling us what's going on would be nice."
One angry passenger told me the 80-year-old man next to him had no hat, was getting very cold and needed help. He was put in a taxi by railway officials.
Another couple told me they'd given up and were going back to their daughter's house.
But others were determined to stick it out - taking turns to get hot drinks and food. Some said they simply didn't have the option of delaying their journeys because they had nowhere to sleep tonight.
The rail operator, East Coast Trains, apologised for the disruption and said tickets would be valid on Sunday services.
Its spokesperson said: "Passengers travelling to and from London King's Cross are advised to travel on another day if at all possible."
Finsbury Park station was closed for safety reasons for an hour from 11:00 GMT as "significant crowds" had been gathering on the platforms.
After the delays at Finsbury Park, East Coast said its "strong advice" to passengers was to defer travel to another date. Saturday's tickets will be valid on Sunday and Monday.
For passengers who travel East Coast said it was operating a service every 30 minutes from Finsbury Park - all trains run to Peterborough, then continue either to Leeds, Newcastle or Edinburgh.
Great Northern, which also uses King's Cross also advised passengers not to travel on its services on Saturday until further notice.
West Coast Main Line services are also not running between London Euston and Hemel Hempstead in Hertfordshire until 29 December, due to engineering works.
Transport Secretary Mr McLoughlin said: "The situation on the railways this weekend has been totally unacceptable. Passengers must be able to trust that vital engineering works on the rail network will be completed on time.
"I will be asking Network Rail to set out what went wrong and how they can learn lessons, but its priority must be to get services running into Kings Cross as well as Paddington."
Labour, meanwhile, accused the government of allowing almost the entire rail network to be shut down on Boxing Day.
Shadow transport secretary Michael Dugher said: "Now we see this further unacceptable disruption, just as people try and get home after Christmas."
BBC journalist Vicky Riddell said there had been chaotic scenes at Finsbury Park.
"People were unable to leave the station via one of the exits due to overcrowding in the underground passageways which led to the overground platforms," she said.
"Outside, the entrance to the mainline station was blocked due to the volume of people trying to get in and out, leading to people missing their trains due to being simply unable to get into the station."
Jonathan Nicholls, from London, had queued for two hours outside the station to try and get a train to Berwick-upon-Tweed.
He said: "There's no-one outside the station giving any directions, no police or anything."
Passengers arriving at Peterborough also described the scene at Finsbury Park as "chaos" and told the BBC they had waited about two hours before being put on a train. Some said too little information had been given out but a woman travelling with her baby daughter said station staff had been "fantastic".
Passengers at Reading also described their difficult journeys, with one man wondering why the announcement about the engineering work was not made earlier, and a woman saying she had to arrange for a taxi to take her to Gatwick airport.
David Sidebottom, passenger director at the watchdog Passenger Focus, said: "Investment in maintenance and improvement is necessary, and we passengers understand that.
"We will be looking to see that operators and Network Rail are doing all in their power to alert passengers, to help them make alternative arrangements and to make it easy for them to claim refunds or compensation."
A Network Rail spokesman said the work was "a small part of a massive amount of engineering investment taking place over Christmas".
He said 4.5 million passengers use the railways on average every day, compared with two million a day over the Christmas and New Year holiday.
About 300 projects are being undertaken over the holidays across 2,000 sites up and down the country.
National Rail said a reduced service to and from London King's Cross was expected to operate on Sunday, but journeys could be retimed and take longer than expected.
Trains will leave King's Cross up to 20 minutes earlier than normal, resuming their usual stopping times from Peterborough. Services to King's Cross will arrive up to 40 minutes later than normal.
Other services will start or end at Doncaster, Newark North Gate, Peterborough or Stevenage.