I recall seeing a show on Discovery over a year ago about terrorism, including various biological and nuclear threats. It turns out that in Russia has "lost" several "suitcase nukes" about five years ago. They are about the size of a backpack, loaded with 1 Mton of fission power (the same as the atom bombs used to attack Hiroshima and Nagasaki). Imagine if someone took a subway train to Washington D.C. from far away, set the timer to one hour and then left the train about one hour before its arrival, leaving the backpack behind. How often do you really think people check out forgotten backpacks on trains while they're moving? Not very often.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram