Question: On Thursday the London assembly's transport committee is taking evidence on the latest derailments from London Underground, Tube Lines and is hoping to hear from Metronet. Does this indicate you can no longer be clear that one person or one organisation is responsible for safety on the Tube?

John Biggs: The legal responsibility for safety lies with London Underground, but obviously it is important to get to the bottom of who was really responsible for the maintenance which appears to have gone wrong on this occasion.

Question: Are you concerned that the system is more fragmented now than it should be?

John Biggs: I think it is important that a person in my position should keep a reasonably open mind on this because the consequences of whatever conclusion we reach could be pretty far reaching.

It is tempting to leap to conclusions, but we don't really know. For example, the press has suggested that the defects might have been there for several months, and the inspection regime hasn't changed since the PPPs went ahead.

So I would like to get to the bottom of that and understand whether they have been doing anything new which has inadvertently created a risk, whether they are cutting corners, whether they know their contractors, whether there is casualisation of staff.

They are good questions to ask and I don't know the answers to them yet.

Question: You led the committee's investigation into the Chancery Lane derailment. Do you believe there were lessons to be learned from that which haven't sunk in yet?

John Biggs: The conclusion we drew from that was summarised in the title of the report, which was "An accident waiting to happen".

What we concluded on Chancery Lane was there was a defect which everyone know about which hadn't been properly addressed, namely the problem with the trains and the engine falling of them. Sooner or later it was going to happen in a way that was dangerous.

We don't know enough about the maintenance of these tracks yet, whether anything has changed since the beginning of the year.

Question: Mayor Ken Livingstone has been quick to blame the PPP.

John Biggs: Of course, he would. I've got a lot of time for Ken on some areas but I think he has been prematurely opportunistic.

Question: Have the rows over PPP given added significance to an event that might previously have been seen as a relatively minor derailment?

John Biggs: I think they certainly have, yes. I meet the PPP contractors reasonably regularly and they know that they have got a job on to win public confidence. They know also that without public confidence their share prices will go down the plughole, so it is not as if they don't have any self-interest in this.

There is a very clear and critical interest in them providing a good service. If they don't then they will suffer as much as other people.

Question: Recent events have not been good for the Tube, are they symptomatic of wider problems perhaps stemming from a lack of investment?

John Biggs: On the face of it, it is a very safe system and has been for many years. There is nothing yet to suggest that should be substantially called into question. If there was a further accident there might be far more difficulty in saying that. It is too early to say what the final conclusions are.

But what is needed is a very quick and clear conclusion from the investigation that reassures public confidence.

Question: Can the committee play a role in that?

John Biggs: Yes, absolutely. That is what we have done before and that is what we are elected to do. We are there to represent the concerns of ordinary Londoners who are not technical experts but have very real concerns about the safety of their journey to work, particularly as for many people there is no other alternative.

Question: Can the committee force any action to be taken, or will it just right a report that nobody is compelled to act on?

John Biggs: We can be influential. In the end the rail safety bodies will have to make whatever hard recommendations they make but we are in the business of expressing the public interest in these matters.

If there are obvious lessons to come out of it then the rail companies would be reckless not to take them into account.