Seven people have been injured in the second derailment on London's Tube network within 48 hours.
The injured were taken to hospital
About 200 people were moved to safety after a carriage left the tracks and hit a wall as it pulled into Camden Town station in north London just after 1000 BST on Sunday.
London Underground believes the sixth car came off the line and detached from the rest of the Northern Line train, possibly because of a fault with the track.
Questions have been raised about safety on the Tube.
Bob Crow, general secretary of the RMT union,
threatened industrial action unless maintenance contracts given to private
companies earlier this year were immediately suspended.
Five of those injured were discharged from hospital after treatment for minor injuries and two were detained.
One of those has a broken thigh and another is being kept in for observation after suffering a minor head injury, said a spokeswoman for the nearby Royal Free Hospital.
Camden is one of London's busiest stations at the weekend because of its popular markets.
The 70 passengers who were on the train were led to safety, as were another 140 or so on two trains stuck behind it.
Eyewitness Harry Anscombe, 22, arrived at the station at 1015 BST as emergency services pulled up outside.
"There were lots of blackened faces and there was one man with bandages on his face and blood streaming down," he said.
"They had lined up some chairs at the top of the escalators and were sitting people down with blankets around them."
The accident follows a low-speed derailment near Barons Court station in west London on Friday night.
It is thought a rail broke beneath a Piccadilly Line train carrying 76 passengers as it came out of Hammersmith station.
London's Transport Commissioner Bob Kiley said: "We have to get to the bottom of these two incidents and we have to take steps to ensure that these things cannot happen again.
Tube derailments 2003
25 January: Chancery Lane crash on Central Line injures 32
17 October: Barons Court derailment on Piccadilly Line. No casualties
19 October: Camden Town derailment on Northern Line. Six injuries
"The average person is going to think: 'Can I really use the Tube if two of these things happen within two days?'
He said both Transport for London and the private firms which took over the maintenance of the Tube in January would have to answer that question.
Aslef, which represents train drivers, said there had also been concerns about signal failures in Camden in recent days.
London regional official Steve Grant said he had "serious concerns" about
the maintenance regime on the Underground.
The RMT's Bob Crow said train drivers had recently warned of problems with the track.
He added: "Track walks to check the tracks
used to be done every day. Now they are only done once a week."
London Underground (LU) apologised to passengers and said a full investigation had begun.
Safety director Mike Strzelecki said early indications were that the problem was with the track rather
than the train.
Camden station remains closed until further notice, with the Northern Line severely disrupted.
There is no service between Charing Cross and Hampstead on the Charing Cross Branch, or Kings Cross and East Finchley on the Bank branch.
Between 60 and 70 extra buses have been laid on for the stops between, but LU is advising passengers to leave extra time for their journeys.
Passengers can check latest updates on LU's travel information line on 0207 222 1234.
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