London Underground is to be prosecuted by the rail regulator over a runaway engineering train, it has been disclosed.
The incident happened on the Northern Line at the start of the morning rush hour in August 2010.
Maintenance firm Tube Lines is also to be prosecuted by the Office of Rail Regulation.
Mike Strzelecki, London Underground's Director of Safety, said: "This was clearly a serious incident and London Underground and Tube Lines acted quickly to investigate its causes and take action to ensure that the risk of such a rare incident recurring was minimised.
"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 and Tube Lines have put in place even tighter approvals and controls for the design and use of all such equipment.
"LU staff's swift actions meant that this incident was drawn to a safe conclusion. London Underground has an excellent and improving safety record, with more than ten times fewer serious incidents on the Tube than in 2000. This has been achieved in part by ensuring that all significant incidents are investigated thoroughly and the root causes addressed.
"We are very disappointed with the ORR's decision to prosecute both London Underground and Tube Lines in this case, especially as we have already implemented those recommendations made by the independent Rail Accident Investigation Branch. This was in addition to recommendations made and acted on from our own report into the incident published in November 2011."
Bob Crow, leader of the Rail Maritime and Transport union said: "This was an horrific incident that came perilously close to a grinder colliding with a passenger train and RMT warned at the time that it was a serious mistake on the part of London Underground to downplay the potential consequences.
"The very least we would expect now is for LU to rule out any suggestion that Tube Lines should be re-privatised, to take all contracted work fully in-house with staff integrated on LU terms and conditions and we want an assurance that the cuts programme, and the associated pressure on staff to cut corners, should be brought to an immediate halt."