'Flaws' led to runaway Northern Line Tube train
A series of flaws led to a runaway train on the London Underground that could have ended in fatalities, a Transport for London report concluded.
The train went out of control on Friday 13 August, passing several stops. It came as close as 500m to a full train.
Vehicles in front were ordered not to stop at stations to outrun the vehicle and avoid a collision.
Now a TfL report has blamed a faulty coupler and excessive speed by a rescue train for the incident.
Trouble began when a rail grinding unit - used to remove imperfections in rails - broke down at Archway station in the early hours.
An out-of-service passenger train was then sent to pick it up and the two trains were connected with an emergency coupler.
The vehicles began to head north.
But as the vehicles left Archway Station, the rescuing train "exceeded the restricted speed of the recovery" (10.2mph), the report said.
The inquiry claims this led to an emergency brake system kicking in and the resulting force broke the coupling between the two trains.
Gravity then sent the rail grinding unit heading south.
An e-mail sent by Transport for London management, and obtained by BBC London, shows the vehicle reached a top speed of 32mph on its dash past Kentish Town, Camden Town, Mornington Crescent and Euston.
A highly-placed source has told BBC London two vehicles were just 49 seconds apart during the incident.
The report makes clear: "The potential outcomes of the incident were a collision with another train or with the infrastructure, either of which might have resulted in serious injuries or fatalities."
As the train rolled "without control", a last ditch attempt was made to derail it with signal points, but this also failed.
The report also reveals the same rail grinding unit had broken down a month previously at West Hampstead Station.
It criticises "flawed emergency coupler design", "flawed approvals processes for the emergency plan" and "time pressure for service resumption affecting decision making".
A TfL spokesman said: "This was clearly a serious incident and London Underground has acted quickly to investigate its causes and prevent this from happening again.
"Following the incident LU immediately put in place procedures to remove the engineering train and tow-bar from use on the railway.
"Since then LU has put in place tighter approvals for the design of the towing equipment."
He continued: "All staff involved followed LU's procedures correctly, and their swift actions meant that this incident was drawn to a safe conclusion."
Speaking after the incident, passengers on the train in front of the grinding unit described their terror as they were told to go to the front of the vehicle, which picked up speed and did not stop at stations.
Tom Redfern said: "The driver came on the tannoy and said, 'There is an emergency, will everyone move towards the front of the train'.
"There was a ripple of panic. I went from half asleep to a big adrenaline rush. I thought, 'Is this it?'"
The Rail Accident Investigation Branch (RAIB) is conducting its own investigation.