Al-Qaeda attempts to bomb Metro were foiled when:
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AQ Bomber # 1 arrives at Woodley Park-Zoo station and discovers the escalator is out-of-order. Bomb kills him before he can get all the way downstairs.
AQ Bomber # 2 arrives at Smithsonian station and finds it clogged with tourists who are blocking the whole escalator. He doesn't know how to yell "stay to the right". Bomb kills him as he's running to the farecard machine.
AQ Bomber # 3 makes it to a farecard machine. The farecard machine won't take either of his bills or half his change. Bomb goes off and kills him as he fumbles for another nickel.
AQ Bomber # 4 decided to buy farecard with his debit card. Bomb goes off and kills him before he can figure the freakin' thing out.
AQ Bomber # 5 makes it through turnstile only to find the last esaclator is out of order and nobody is moving. Bomb goes off and kills him as he runs towards the train (bing! doors closing! bing!).
I'm not sure Metro needs more security.