A runaway train brought chaos to London Underground yesterday as it hurtled through six stops on the Northern line.
Staff were forced to divert other trains from the path of the unmanned engineering train, which rolled south for 13 minutes before coming to a stop at Warren Street station.
London Underground said the train had become defective at 5.25am yesterday as it approached Archway station. Engineers began to move the train northbound by coupling it to an out-of-service Northern line train.
A spokesman went on: "However, for reasons that are now under investigation, at around 6.44am the engineering train became detached from the Northern line train and began to move southbound."
As staff in the Underground's central control room monitored the runaway car's progress, trains filling up before the morning rush-hour were diverted to the Northern line's City branch, while the engineering train was diverted along the parallel Charing Cross branch.
Services on large parts of the Northern line were suspended until late morning. Passengers on some trains were told the disruption had been caused by "late-running engineering works".
But as details of the incident emerged yesterday afternoon, transport unions expressed concern. Pat Sikorski, assistant general secretary of the RMT transport union, said: "We are appalled and horrified at this major incident during passenger traffic hours and 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."
Mr Sikorski said the RMT was "seeking urgent answers as to whether or not third-party contractors were involved in this incident – particularly as the findings from Potters Bar [rail crash] ruled that third-party contractors should not be involved in rail maintenance works".
He went on: "This horrific failure comes against a background of a systematic reduction in safety-critical jobs and safety procedures as a direct result of Transport for London's cuts programme. In the light of this morning's events it is essential that TfL call a halt to their cuts plans and bring an end to the dilution of Tube safety."
The LU's director, Richard Parry, said: "Safety is our top priority, and we have of course launched an immediate and thorough investigation into this incident to establish the cause.
"Once that investigation is complete, we will publish the report, making its conclusions and recommendations clear. In the meantime, we have prohibited the use of this design of engineering train on the Underground.
"From the start of this incident, an immediate assessment was made and operational decisions taken to minimise the safety risk to our customers and staff."