From the tragedy in Moscow's subway, to the 2004 Madrid train bombings and the explosions in London's public transit system the following year, experts say no country is immune to terrorism.

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But if you look at the terrorist incidents since then, particularly the London subway bombings, the two bombings of July of last year, the Russian airplane bombings, even the Indian train bombings, they haven't had a terrible effect on U.S.-Europe air travel.

NPR: Airlines Take Security Alert in Stride