THE Rail Accident Investigation Branch and Transport for London have launched a joint probe after an engineering train ran away on the Northern Line early on 13 August, triggering an emergency operation to prevent a possible collision.

Transport for London said that around 05.25 an engineering train developed a fault 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 06.44 the engineering train became detached from the rescue train and began to move southbound.

London Underground then took emergency action to clear the Charing Cross branch. Drivers were ordered to run their trains out of the section without stopping at any intermediate stations. Meanwhile, approaching trains were diverted to the City branch.

With services on the Charing Cross branch suspended, the engineering train ran south through Camden Town and came to a stand at Warren Street at 06.57, having made an uncontrolled journey of some 6km.

Engineering trains of this type, which are operated by the former public-private partner Tube Lines, have been withdrawn from service while the investigation continues.

Tube Lines was recently taken back into public ownership by Transport for London's purchase of all the shares in the consortium for £310 million, but still operates as a separate unit.

Services between Finchley Central and Archway, and also Camden Town to Kennington via Charing Cross, remained suspended on Friday afternoon but were restored later in the day, with only 'minor delays' still being reported at 19.30.

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.

“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. We apologise for the inconvenience caused.''

¶ The incident was the second affecting a London Underground engineer's train in the last three months. The RAIB is also currently investigating the derailment of a battery locomotive between Gloucester Road and Earls Court early on 12 May. No one was hurt, but the locomotive and track were damaged.