An investigation has begun into how a train carrying hundreds of passengers split in two on Thursday night while travelling at 145km/h (90mph).
First Capital Connect runs the service out of King's Cross
About 700 people are thought to have been on the 1707 from King's Cross to Peterborough when it came to a sudden halt eight minutes into its journey.
The First Capital Connect train, made up of two four-car units, "decoupled" and both halves came to a halt.
A train company spokeswoman said passengers were never in danger.
She said: "It was travelling at 90mph but the trains are designed for such things to happen at those speeds and to stop safely."
She said the carriages had ended up stopping less than a carriage-length apart.
'Communication cut off'
Passenger Roslyn Magen, 57, said it was "the strangest" problem to affect the line so far.
Mrs Magen works for Lloyds TSB at London Bridge but lives near St Neots in Cambridgeshire.
She was in the second unit, to which communication with the driver was cut off.
"We sat there and sat there and there was no announcement or anything. Everybody was getting agitated because nobody was telling us what was going on."
Customer services first told her there was a "technical problem" at New Southgate, but as trains continued to pass them on the slow track, she phoned back 20 minutes later.
She was told the train had "decoupled".
"At first they (the passengers) were annoyed because there was no announcement, but once they realised what it was they saw the funny side of it," she told BBC News.
"There was no announcement because there was nobody there."
About 45 minutes later a replacement driver arrived to get the units moving again.
Larry Heyman, from First Capital Connect, said it was investigating why the coupling mechanism had failed, as the units were new stock, adding that it was very rare.
He said there was no danger of another train running into the carriages as safety systems would show they were on the track.
Both units came to an immediate halt when the carriages split, he said.
But he added: "We really would like to apologise for this, it's something that concerns us immensely - these particular units are quite new.
"Our number one concern is customer safety and the safety of everybody using the line."
The results of the company's investigation are expected to be revealed next week.