Rail expert Christian Wolmar answered BBC News Online's questions about the crash.

How might this have happened?

In derailments there are really two main possible causes. One is that somehow the rail broke underneath. This seems fairly unlikely as the rail was near the station and this track is fairly easy to maintain. Otherwise there could have been an obstruction on the track but this would have normally derailed the front of the train not the rear three carriages.

The other thing is that it could be something wrong with the axle of the train, this might explain the smoke. The axle might have malfunctioned in some way. Now the axles are maintained very well and they are unlikely to be the explanation, but then again there has to be an explanation for what is a very unusual event.

How lucky were the passengers that the train was going fairly slowly?

There is a good element of luck but the Tube system is inherently pretty safe because even if there is a derailment the train has nowhere to go because it is enclosed in a tunnel.

Why was there so much soot?

There would have been much debris bought down. A fire would have been unlikely because there isn't much to burn on these trains. Fire incidents have gone down since the King's Cross.

What is the Tube's safety record like?

The Tube has an excellent safety record. There are very few major incidents. The last one was King's Cross and before that Moorgate, but apart from those two there hasn't been a major incident in the 140-year history of the whole system. Very good given the frequency of use and the age of the equipment.

But this will be investigated very thoroughly and will be a cause of great concern. I think it will raise a lot of issues, especially with the private-public partnership coming in.