LU said in a statement that at around 5.25am an engineering train performing rail maintenance became defective as it approached Archway station. The train was travelling southbound on the High Barnet branch of the Northern Line.
Engineers began to move the defective train northbound by attaching it to an out-of-service Northern Line train.
For reasons that are now under investigation, at around 6.44am the engineering train became detached from the out-of-service Northern Line train and began to move southbound.
LU staff diverted passenger trains to the City branch of the Northern Line while directing the engineering train to the Charing Cross branch. Passenger services on the Charing Cross branch of the Northern Line were then suspended.
The engineering train eventually came to a halt at Warren Street station on the Charing Cross branch of the Northern Line at 6.57am.
LU director Richard Parry said: ’Safety is London Underground’s 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.’
RMT union assistant general secretary Pat Sikorski said: ‘RMT is appalled and horrified at the major incident that occurred this morning 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 as a broken down grinder unit was being pulled northbound at 6.40am this morning, represents a safety failure of the highest order. We understand that a collision with a passenger service train leaving Archway was only narrowly avoided.
‘The overnight works were the responsibility of TfL subsidiary Tube Lines, managed by the private company Amey, and RMT are seeking urgent answers as to whether or not third-party contractors were involved in this incident – particularly as the findings from Potters Bar ruled that third-party contractors should not be involved in rail maintenance works.
LU says it has suspended the use of all engineering trains of the type involved in the incident, which are provided to LU and operated by a contractor, until further investigations can be undertaken.
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