A runaway train went through six stops on a 13-minute journey on London's Tube with other trains having to be cleared out of its path.
The engineering train became uncoupled as it was towed on the Northern Line near Archway station on Friday morning.
Passenger trains were diverted to another branch of the Northern Line while trains were cleared from the Charing Cross branch.
The train ran for nearly four miles before it stopped at Warren Street.
It came to a rest only because there is a slight incline at the station.
Pat Sikorski, assistant general secretary of the Rail, Maritime and Transport union, said he was "horrified" as it could have "very easily resulted in disaster". He said the union understood a collision with a passenger train "was only narrowly avoided".
The train was stopped by a "slight incline" at Warren Street station, London Underground director Richard Parry said.
London Underground (LU) said the engineering train, which was undertaking rail maintenance work, had been travelling southbound on the High Barnet branch of the Northern Line.
A statement said the train became defective as it approached Archway, in north London, at about 0525 BST.
Engineers began to move the train northbound by coupling it to an out-of-service Northern Line train.
But at 0644 BST the engineering train, which does not carry passengers, became detached from the towing train and began to move southbound.
LU staff took the decision to divert passenger trains to the City branch while directing the engineering train to the cleared Charing Cross branch.
The engineering train came to a halt at Warren Street in central London at 0657 BST, only stopping because the station is at the bottom of an incline.
LU's Richard Parry said he "could not speculate" about how fast the train had been travelling.
"Northern line trains will travel even at full propulsion at 35mph," he added.
"I'm confident it would have been at a lower speed than that."
He said no passenger trains were closer than a kilometre (0.6 miles) away from the runaway train.
"It was no closer than a kilometre away. That was the closest any train was in the area to where this train was," he said.
Mr Parry said that the train was stopped by a slight incline in the track.
"There's a slight incline coming out of Warren Street station," he said.
"We had attempted to slow the train down as it was coming through Mornington Crescent and it then came to a halt."
The line was suspended between Finchley Central and Archway and between Camden Town and Kennington via Charing Cross, leading to widespread disruption.
Mr Parry added that: "Safety is our top priority and we have, of course, launched an immediate and thorough investigation into this incident to establish the cause."
Mr Sikorski, from the RMT, said: "We are appalled and horrified at this major incident during passenger traffic hours which could have very easily resulted in disaster.
"The runaway train, which it is suspected broke loose from a failed emergency coupling, represents a safety failure of the highest order. We understand that a collision with a passenger service train leaving Archway was only narrowly avoided."
He added that the union was "seeking urgent answers" as to whether third party contractors were involved.
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