Following the second Tube derailment in 48 hours, rail expert Christian Wolmar gives his opinion of the issues involved.
Sunday's derailment on the Northern Line comes less than 48 hours after another derailment, on the Piccadilly line. What explanations might there be?
Apparently the Piccadilly Line accident on Friday was caused by a broken rail which had been checked quite recently.
Broken rails do happen, but normally the maintenance regime will spot them before they happen.
This derailment might have been caused by a broken rail, or it could have been something like the Chancery Lane accident [in January 2003].
Then, something came off the train and caused the last three carriages to derail.
What part might privatisation have played in this incident?
London Underground itself remains in public hands but maintenance has been handed over to contractors.
The maintenance of the Jubilee, Northern and Piccadilly lines was handed over to Tubelines - made up of Jarvis, Amey and Bechtel - and Metronet.
So coincidentally, perhaps, that company has been concerned in both of those recent derailments. This will be a great embarrassment.
Jarvis recently pulled out of maintenance on the national rail network because of a series of derailments caused by mistakes made by the company - and this was so bad for their PR they pulled out of maintenance.
On the underground, though, they have a 30-year contract, and they're not about to pull out of it.
Why would maintenance suffer under the privatised system?
The argument is that it is much better to have one company in control of the whole operation of the Tube.
That would mean not splitting the day-to-day operations from the maintenance of trains and track. You do get problems of communication, and so on.
London Underground will have to look at this very carefully to see whether there is something systematic about the failure.
If the privatised system is found to be at fault, how can the system be changed?
That would be very, very difficult. These private companies have 30-year contracts.
They can only be removed if they have done something very badly wrong. It would be the result of major lawsuits, millions of pounds being spent.
Certainly for the next seven-and-a-half years - which is how long before the first real assessment of the contracts - and probably for the next 30 years, it will be very difficult to change.
What might be the reaction of London Underground managers and those representing drivers on the Tube network?
The unions will be up in arms, and among the management there will be intense fury.
They didn't want this public-private partnership - the government pushed it through. There was huge controversy over that.
Managers thought this system would be very difficult to work effectively. This will clearly provide further evidence for that view.
What effect do you think this have on the travelling public in London?
I'm concerned there will be panic and people will take to their cars - that would cause chaos.
In fact, these are relatively minor incidents. London Underground are very good at coping with these incidents, there has been a long history of improvements since the King's Cross fire in 1987.
There have only been two major disasters in the peacetime history of the tube.
It is a remarkably safe network, carrying three million people a day. It's still so much safer than the roads.