A full investigation has been launched into an incident involving a runaway train on London's Tube on Friday morning.
The Rail Accident Investigation Bureau (RAIB) said it could not say how long it expected the investigation to take.
The engineering train went through six stops on a 13-minute journey on the Northern Line with other trains having to be cleared out of its path.
The train became uncoupled as it was towed near Archway station.
Passenger trains were diverted to another branch of the Northern Line while trains were cleared from the Charing Cross branch.
The train ran for nearly four miles before it stopped at Warren Street, in central London.
On Friday London Underground's strategy director Richard Parry said a preliminary investigation had been launched.
He said the train came to a halt because of a "slight incline" at Warren Street station.
A London Underground spokesman said the preliminary investigation was "likely to take several weeks".