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In the aftermath of the Moscow Metro suicide bombings, Russian President Dmitry Medvedev has called for tighter security measures on public transport. Transport security analyst Chris Yates examines whether it is possible to secure large networks against the threat of terrorism.
It is currently very difficult to prevent a suicide bomber from attacking a public transport system.
Airport-style security cannot be instigated in a transport operation like the Moscow Metro or indeed the London Underground. It would bring systems to a halt.
The Moscow Metro has five million or so passengers per day - it is just impossible to process that number of people with the kind of security that is in place in an airport.
What can be done is to have a much greater degree of surveillance, using those ubiquitous CCTV systems, some of which are very good nowadays - the new digital systems, for example.
This makes it possible to monitor people moving about in the transit system, and where operators pick up unusual activity, law enforcement agents would be able to focus on whoever was acting suspiciously and mitigate whatever the threat might be.
This is not easy, as it requires constant monitoring of CCTV. There are systems that allegedly will automate some of the analysis process, but the jury is still out on whether these automated systems have any utility.
There has been some talk, in the hours since the Moscow bombing, about behavioural analysis, which might be useful, but this remains to be seen.
In such a high-pressure transport system, to try to pick out individuals based on behaviour is not going to be easy.
One only needs to sit in a cafe at a railway station to see every type of individual go past in the course of an hour.
Anyone in mass transit security will agree on the fact that there is not a lot of technology out there at the moment to stop a determined suicide bomber.
Facial recognition requires all the right conditions to function.
Most require that images being captured are compared against a database, so a high-quality image is needed to start with to do the match. In the case of some terrorists, that image will not exist.
There is some advanced technology which could have some utility in the future: a scanning device being developed at Loughborough University, called an explosives residue detector, uses a UV light source to scan individuals as they enter the underground network.
It will give a clear indication if somebody has handled explosives and if they have, this gives an indication that they may be wearing explosives.
This technology is slowly coming to commercialisation. People are not asked to stop - it is a non-contact system and it will scan a person in a couple of seconds and raise the alarm.
Of course, there has to be some method of responding to the alarm.
Given the risk to mass transit systems these days, there is a much greater police presence - this applies in cities like London, Moscow or New York - so it is a question of focusing the attention of law enforcement agencies on a potential threat.