In the wake of the subway bombings in Moscow, public transport systems seem vulnerable. London's system is one of the largest and most complex in the world. Officials in the British capital have long worked to keep the transport system safe, but have not always succeeded.
In July 2005, suicide bombers attacked three London underground trains and a bus killing 52 people, including themselves.
Two weeks later, four more would-be bombers tried, but failed to attack underground trains. Their bombs did not explode.
Analyst Jonathan Wood with the security firm Control Risks in London, says public transport systems are an attractive target.
"An attack on a public location, but particularly one that people use in their everyday lives, has a very strong psychological impact and it also tends to generate a lot of media coverage," he said. "So it draws attention to the terrorists groups and their aims and their goals."
Al-Qaida has already said it's focused on public transport, according to this video featuring an al-Qaida spokesman, the American Adam Gadahn.
GADAHN: "We must keep in mind how even apparently unsuccessful attacks on Western mass transportation systems can bring major cities to a halt, cost the enemy billions and send his corporations into bankruptcy."
London may be famous for its red double-decker buses, but it is the underground system, parts of it more than a hundred years old, that is especially vulnerable.
The city has more than 10,000 closed circuit security cameras, including in the transportation system. Software can be added to identify things that are not usually there, like a bag left on a bench.
But analysts say public transport is hard to police. Telling the difference between a regular commuter and an attacker is difficult on camera or in person.
Nigel Inkster, a terrorism expert at the International Institute for Security Studies says the 2005 London bombings proved that the city's transport network is vulnerable.
"At least one of the bombers detonated in the deepest part of the underground which made it hugely difficult both to undertake rescue operations and to carry out the forensic examination of the site which needed to take place," said Inkster.
Authorities here depend on the public to keep an eye out for unusual packages or passengers.
And London's Transport Authority has promoted the Oyster card, a magnetic payment system that keeps track of where people have been and when. But that is only helpful in surveillance or a post-incident investigation.
"I don't think you can look at public transportation in isolation," Inkster added. "You've got to look at it as part of a comprehensive package of counterterrorism measures."
With London hosting the 2012 Olympics, security is a major concern. New transport links to the Olympic village are being built.
While new stations that minimize bomb damage are feasible, officials say the best defense is identifying attackers before they can strike.
