Runaway train on London Tube's Northern Line
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.
Tom Redfern joined the line in Archway on the train in front of the runaway vehicle.
He said: "As soon as we pulled away the driver came on the tannoy and said, 'There is an emergency, will everyone move towards the front of the train'.
"There was a ripple of panic. I went from half asleep to a big adrenaline rush. I thought, 'Is this it?'"
The train bypassed all stations until Moorgate in an attempt to keep ahead of the other vehicle.
Mr Redfern said: "We went full speed. We knew the situation was dangerous because we were going fast.
"Even by the driver's voice, we could tell it was serious."
Pat Sikorski, of the Rail, Maritime and Transport union, said: "We are appalled and horrified at this major incident which could have very easily resulted in disaster.
"The runaway train represents a safety failure of the highest order.
"We understand a collision with a passenger service train leaving Archway was only narrowly avoided."
The engineering train, which was undertaking rail maintenance work, had been travelling southbound on the High Barnet branch of the Northern Line.
But at 0644 BST the vehicle, which does not carry passengers, became detached from the towing train and began to move southbound.
London Underground (LU) staff diverted 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 0657 BST.
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."
Safety 'top priority'
He said no passenger trains were closer than a kilometre (0.6 miles) 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.
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: "Safety is our top priority and we have launched an immediate and thorough investigation into this incident."