London Underground (LU) has admitted it does not know when it can fully restore services on the Northern Line after last Sunday's derailment.
Seven people were taken to hospital after the Camden crash
LU bosses had hoped to reopen Camden Town station, the scene of the derailment, on Monday following repairs to cables and track damaged in the crash.
But LU managing director Tim O'Toole told BBC London's political editor Tim Donovan on The Politics Show that the work was taking longer than expected.
Seven people were injured at Camden, north London, on 19 October when the rear carriage of a Northern Line train came off the track and hit the wall.
"We're now finding out that Tube Lines engineers are running into difficulty with some of the signal cables they've had to restore," Mr O'Toole said.
"The work's taking much longer than they expected. We're reassessing the situation now but I think it highly unlikely we'll restore service in the morning.
"I'm going to revisit this with our engineering forces and we will come up with more reliable information when it's available."
Mr O' Toole also revealed that it was not yet known what caused the train to derail.
"It's extremely difficult to determine exactly what occurred there.
"We've brought in an expert outside consulting firm who's doing some modelling to try to determine the forces that would have allowed this wheel to climb the rail."
The Camden Town incident came 48 hours after a Piccadilly Line train derailed at Hammersmith. A cracked rail was blamed.
Ballot papers are due to go out next week to members of the Rail Maritime and Transport (RMT) union over possible strikes and "go slows" following the derailments.
RMT general secretary Bob Crow said drivers would be asked to reduce speeds as part of a campaign of industrial action which would cause "complete chaos" to Tube services in the run up to Christmas.
Drivers could be told to slow down to just 20mph.
The union has been called to a safety summit with LU on Tuesday, but Mr Crow said he was not prepared to tolerate any compromise on safety.
Mr O'Toole said on Sunday that LU was still looking at maintenance regimes and practices and was considering more use of ultrasonic technology to check for faults in the track.