http://www.bbc.co.uk/news/uk-england-10964766

A driverless train ran for almost four miles on the London Underground
on Friday morning.

The engineering train on the Northern line was allowed to roll between
Archway and Warren Street stations, TfL has confirmed.

The incident, which happened at 0640 BST, is now being investigated by
London Underground.

Transport for London (TfL) said the train moved at slow speed with no
risk to the safety of passengers or staff.

A TfL spokesperson said: "A full investigation has been launched into
this incident."


So this is why the Northern Line was, and is still suspended. It was
described as "late finish to overnight engineering work"