The talk now regarding terrorists using the mass transit systems has been focused on bombs, which is only natural since this has been the modus operandi for both Madrid and London.
But let us not forget what happened on March 20, 1995 in the Tokyo subway system.
A religious group called Aum Shinrikyo coordinated a five-prong attack where members entered differing subway lines at the same time, armed with plastic pouches of sarin, in liquid form, and sharp-pointed umbrellas. The plan was for each member, upon exiting the train at a set time, to puncture the plastic pouches, releasing the sarin gas.
Sarin was developed by the Nazis and is one of the most deadly nerve agents known.
“Only” 12 people were killed, but as many as six thousand were injured, many severely, and the subway line was put into a panic and shut down.
Casualties probably should have been much higher. Not all of the five were successful in puncturing all of the packets. Some subway workers, to their detriment, were also quick to clean up what they thought was either spilled water or a harmless liquid.
If such an attack were to take place in London or New York, they would be very difficult to prevent, especially by random bag checks (the liquid could be contained in water bottles, for example.) The question is do the anti-British, anti-American terrorist have access to these types of agents?
Aum Shinkrikyo was “blessed” with some very highly educated members, including many scientists. They also had access to large funds. They were able to build their own chemical lab and create the sarin. This is not an easy task; not so much because of the difficulty of creating the sarin but because of the volatility and extreme toxicity of the substance- it’s much more likely that the persons making it and handling it will die before they are able to deliver it to a target.
Sources: Haruki Murakami’s book “Underground” detailing the attack and the Wikipedia entry, which is very detailed.