Quote from Ame -

Isn't that just a bit melodramatic? Old ladies love public transportation in major metropolitan areas.

According to Wired News, flu, hernia, or police are more likely to kill you than terrorists.
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Yeah, more people die from bee stings...

Nevertheless, if I wanted to close down Philadelphia, I could do it with four people: one each with a firebomb (see note below) for the (1) Market Street and (2) Broad Street subways, (3) Independence Mall, and (4) a commuter train in the Septa Tunnel between Market East and Suburban Station. If you wanted to take it a step further, simultaneous explosions in a few Walmarts and area malls, and a radiological attack in the center of town (few actual casualties, but mass hysteria). All told, maybe a dozen people could pull this off. They wouldn't even have to be suicide bombers - just able to set a timer. This goes for any major city. Just substitute Lincoln Tunnel for Broad street Subway.

Note: A popular "survivalist" website (which will remain unnamed) recently had a post with a link showing, in great detail with diagrams, how to make a firebomb with items readily available to anyone.