London Underground (LU) has launched an investigation into a 'runaway' maintenance train on the Northern line with the Rail Accident Investigation Bureau.
The incident occurred on Friday morning at 5.25am.
According to LU, an engineering train that was undertaking rail maintenance work became defective as it approached Archway, travelling southbound on the High Barnet branch of the Northern line.
Engineers began to move the defective train northbound by coupling it to an out of service Northern line train.
However, 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.
It didn't come to a standstill until reaching Warren Street on the Charing Cross branch, four miles further south, at 6.57am.
LU diverted passenger trains to the City branch while directing the engineering train to the Charing Cross branch where passenger trains had been non-stopped and cleared.
Services on the Charing Cross branch of the Northern line were then suspended.
LU has suspended the use of all engineering trains of the type involved in today's incident, while further investigations are undertaken.
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.”