Parts of the Northern Line could remain closed for several days after a Tube train derailed, London Underground (LU) has said.
The leader of the RMT has called for a strike ballot over safety fears
Buses were laid on to help thousands of Northern Line commuters make their journey to work on Monday after the incident at Camden Town.
The derailed carriage is still in the tunnel and it could take a couple of days to remove as well as carry out checks on the track where signalling equipment was damaged.
The Rail, Maritime and Transport union (RMT) has given Tube bosses a Friday deadline to answer its safety concerns or face a strike ballot.
Seven people were taken to hospital and more than 200 led to safety after part of the train came off the track and hit the tunnel wall on Sunday.
The crash happened less than 48 hours after a derailment near Hammersmith, in west London.
LU's safety director Mike Strzelecki said travellers should try to avoid using the Northern Line.
There are no trains running on the line between East Finchley and Euston and from Golders Green to Charing Cross.
Tube managers are investigating reports that the sixth car was pulled from the rest of the train because of a track fault.
Between 15 and 20 trains passed over that stretch hours before the derailment at Camden.
The Health and Safety Executive (HSE) will examine the track and trains as part of its investigation.
A HSE spokeswoman said on Monday: "Something happened which caused the train to derail and it's going to be very difficult to determine what happened.
"We are going to look at the wheels of the train and how they interfaced with the track."
Five passengers with minor injuries from the Camden crash and one with head injuries have now been discharged from hospital.
One man with a broken thigh is still at the Royal Free Hospital in Hampstead, where he is awaiting surgery.
Sources at the train drivers' union Aslef say members have complained about track problems at Camden Town and Hammersmith in recent weeks.
And RMT General Secretary Bob Crow is threatening to ballot members over industrial action unless maintenance contracts given to private companies this year are suspended.
The union wants a return to daily safety checks on tracks and accuse the private companies of cutting back on checks to cut costs.
Tube Lines, which is responsible for the set of track involved in the Camden derailment, said it is checked every 48 hours.
The investigation will look at why 20 engineers who carried out routine maintenance on the track on Saturday night did not notice any problems.
Tube Lines chief executive Terry Morgan said: "We have obviously got a lot of
thinking and a lot of work to do to understand what caused these incidents.
He added: "We were aware that this particular area needed a lot of work. It
wasn't a safety issue, in fact it was primarily around trying to improve
reliability of that area by increasing the maintenance."
Tube Lines was given responsibility for repairing and upgrading the Northern, Piccadilly and Jubilee Lines this year under the Public Private Partnership scheme.
Is PPP to blame?
London Mayor Ken Livingstone told BBC Radio 4's Today programme he would seek emergency legislation from the government to suspend the maintenance contracts.
He said it could be a result of 19 years of under-investment on the underground but he suspected part-privatisation was the problem.
The HSE's initial findings suggest a broken rail caused the Hammersmith derailment and found the Camden derailment took place over a set of switching points.
Send us your reaction to the tube disruption using the postform below.
My house backs on to the Northern Line at West Finchley, next door to where Harry Beck the original designer of the classic London Underground map lived. A Blue Plaque was erected in his memory in June. He would be horrified to see the tube system in the state it is today.
I urge everyone affected to claim on your charter refund.
The tube system is a joke. It would work better if it was run by a theme park operator. I can't remember the last time I heard of a ride being held up or any of the increasingly desperate sounding excuses used by LU.
Matt Takaira, GB
After 30 years in London, I am pleased to say that I commute to work by car here in Bangalore, India
Naveed Chaudhuri, India
Personally I didn't notice the difference this morning. The Northern Line is always late, overcrowded and unpleasant to use. The sooner I get a job out of London the better.
Every other morning there seems to be a delay on the Northern Line because of problems at Camden Town. What happened yesterday cannot come as a surprise. Expecting an extorted London public to foot the bill for an abysmal sweatbox like the Northern is bad enough. Using their lives as a warning bell for network condition is illegal and disgusting.
Steve Hunt, UK
It doesn't matter who has the overall responsibility for running the Tube: all that is important is that they have plenty of cash and are prepared to spend it repairing the tracks and tunnels, which are in some cases dangerously broken, leaky and in an appalling state. The only reason we have them run by PPP is because the government and the mayor's office were not prepared to spend this money, and all the RMT walkouts and sabre-rattling from Mayor Ken will not change this. Perhaps it's time to spend some of that Congestion Charge money on the Tube, because at the moment all Tube Lines or anyone can do is firefight the immediate problems.
Sarah Bowyer, Reading, UK
My journey every morning is from Finchley Central to Tottenham Court on the Northern Line, then on to the Central Line to Shepherds Bush. This morning I decided not to try joining the scrum to get on replacement buses at East Finchley, opting instead to take the 83 bus service to Marble Arch, two of which passed my stop in 20 minutes full to the gunwales. When I finally did get on a bus, my journey time was extended by an hour-and-a-half, not helped by a road traffic accident that closed Marylebone Road. Are there any extra buses provided on the town-bound services? I doubt it. Sometimes living in London can be fantastic, but this week it sucks. And I fear it will do for some time.
Sometimes living in London can be fantastic, but this week it sucks
Chris Hendrie, UK
Overall the current situation seems to indicate an appalling lack of underinvestment which is being made worse by privatisation. I absolutely agree that profit and safety do not mix and that maintenance of the tube should be in public hands. However, that on its own will still not solve the problems of a Victorian-age infrastructure (much like our sewage and water pipes) in desperate need of not just superficial repairs but a complete overhaul.
Lisa, London, UK
It's no surprise that there's been derailments on the tube, especially when whose responsible for maintaining the tube are more concerned about escalators and the appearance of stations (e.g. Borough and Kennington) than the trains and track. What's the point of having new escalators and attractive stations if the network isn't capable of getting you from A to B?
I fully supported Ken's battle to prevent the privatisation of the tubes, and it looks like he has been proved right. As was clearly seen on the mainlines, making a profit is not compatible with prioritising safety. All maintenance should be brought back to public control, and the government should foot the bill.
Tanya, London, UK (using the Northern Line every day)
How long do we have to wait for something to be done with the London Underground? Is it until someone dies as a result of an accident? Is it when something catastrophic happens along the lines of the Kings Cross disaster? Surely, if we knew that money was going towards the maintenance and repair of the system, we would be happy to pay a little bit extra on tube fares. I for one would be happy to pay a small increase in the knowledge that the money was actively helping to improve the system... better that than family worrying about safety every time something like this happens?
I have sympathy with those that were injured by the derailment, and understand things will take a while to get back to normal - and hope that the incident is investigated fully to ensure that lessons are learned and a repeat in the future becomes less likely. Mayor Livingstone should be working with Tube Lines to get this sorted rather than wasting more energy to try and overturn the PPP, which seems to be his main aim in getting emergency legislation to suspend the contracts.
Brendan Sheehy, United Kingdom
If 1/2 million people use the northern line a day at say an average cost of £2, which makes £1 million taken a day, why can they not afford to inspect the track everyday?
James Johnson, England
On Friday evening of last week I was travelling on my normal route from Bank to Euston, just beyond Kings cross the driver announced that he had been asked to check the track. The train moved off slowly and then slammed to a halt throwing everyone over. It seems to me that LUL suspected that there was a track fault and used a train full of commuters to check it.
Clair Gibson, UK
I would normally be commuting from West Finchley on the Northern Line, and therefore have a vested interest in this. My journey time has been increased by at least 20 minutes each way, probably a lot less than many others. I feel that a lot of money was wasted in the run-up to the Public Private Partnership, both by the Underground and its opponents. What we are now seeing is the result of a culture in which everyone with responsibility is too busy protecting themselves or extending their empires, and putting stewardship of the Underground firmly in second place.
Simon Jackson, UK
Track is designed to last around 10 to 15 years and when rail breaks or joints fail this is not caused by the sudden change of ownership - a car dose not break down because a certain person drives it - this just shows the under investment and lack of maintenance carried out by LUL, as this area must of been in need of repair for some time now.
When are the people of Britain going to organise a protest against the PPP and Jarvis in particular. Have we learnt nothing from Network Rail? The contracts should be terminated and Jarvis ordered to pay costs for endangering peoples lives. Safety and Shareholder profitability do not go hand in hand when most people in this world are essentially greedy.
Steven Walsh, UK
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