The best security is people, both staff and passengers

There are nearly three million journeys on the system daily, and the problems of scanning such vast numbers using the system's 275 stations are legion. There are already 6,000 CCTV cameras on the network, and while more cameras will be coming on stream as the massive station refurbishment programme gets under way there are limits to how many can be actively monitored.

Many stations, built a century or more ago, have no available space for extra controllers, and carving new rooms out of London's clay will cause disruption and cost millions. Besides, CCTV coverage is more of an aid as an investigating tool after events rather than a device to prevent attacks.

Whatever the claims of the security firms, the practicalities of introducing airport-style security on a mass transit system are insuperable. Just to delay each person by 10 seconds would cause chaos at many stations and require expensive technology that would not even be infallible. Stopping people taking baggage on to the system is impractical; as one London Underground spokesman put it, "it is an open mass transit system and has to remain so".

Even low-tech solutions are difficult to implement. London Underground argues, with some justification, that putting guards back on trains would not do much to improve security. They would only be able to sit in one carriage, and even if they were linked through CCTV cameras with other coaches, there would be little they could do in the cramped atmosphere of a rush-hour train.

Ultimately, there is one weapon in the armoury of London Transport and that is people, both staff and passengers. Clearly the staff, who have been widely praised for their efficiency in dealing with the incidents, are far better trained than they were at the time of the King's Cross disaster in 1987 when passengers were directed back on to the burning escalator. Security training is now mandatory and the crowd-control skills of the staff have been very apparent.

Staffing levels at stations have been increased as a result of that disaster and of concerns about security. While that has led to a much higher cost in operating the system, it has also meant that incidents such as the bombings and last week's attacks have been dealt with efficiently.

Passengers, too, have a role to play. Underground managers have been infuriated by the fact that there is still so much lost property being found on the system, leading to dozens of security scares every day in the two weeks since 7 July.

However, as well as simply not leaving things on the Tube, passengers are being encouraged to take a more active role by reporting anything untoward.

Ultimately, better security lies in all our hands.

Christian Wolmar is the author of 'The Subterranean Railway: How London's Underground Was Built and How it Changed the City Forever', published last year by Atlantic Books, £17.99