Killing commuters with bombs has to be one of the most cowardly (and dastardly) forms of terrorism. And that's exactly what happened in Moscow earlier today, as suicide bombers from Chechnya detonated themselves as trains pulled into the Lubyanka and Park Kultury stations. The Russian capital's subway system is one of the busiest in the world, along with Tokyo, New York and London.

Of course, two out of three of the latter cities' systems have already come under attack: deranged cultists released deadly sarin gas in Japan in the 1990s while Muslim extremists detonated bombs across London in 2005.

Unfortunately for New York (and other commuter capitals), such attacks are also almost impossible to stop. After all, the kinds of millimeter wave detectors installed to protect the Green Zone in Baghdad haven't exactly made it a center of safety. And the backscatter X-ray machines used at the White House—and possibly at an airport near you—fail to block party crashers.

Simply put, no technology is foolproof—and all technologies rely on the ability of their human operators. Nor is it economically feasible to put million dollar machines at each of the 180 stations in Moscow's system—or the 468 in New York for that matter—particularly given the fact that once security measures are known and fixed in place, it becomes possible to devise ways to avoid them. Think of the creativity of drug smugglers.

Unpredictable screening is a better idea, as is fighting terrorism at the source: impeding recruitment or pulling up its roots in conflict or poverty. It's also important to remember that while deadly commuter terrorist attacks are a tragedy, they kill tens of thousands fewer people year in and year out than that other common mode of transport: cars. But most people are not afraid to start the ignition in the morning.

Ultimately, the best defense is a good offense. Or, as the slogan goes, if you see something, say something.

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