Six terrorist plots targeting U.S. subway and rail systems have been exposed since the Sept. 11 terrorist attacks, and the systems remain a target, transit authorities, security experts and members of Congress agree. An alleged plot to simultaneously bomb four Washington, D.C., Metro subway stations was foiled in October, and another plot to detonate explosives in New York's subway system was averted last year.[policeone.com]

The problem with even trying to secure the trains, etc. from tangos is that you can get just as big a score from blowing up the location where everyone is being searched as you can by actually hitting the train or bus. Planes are somewhat different because the can be used as a guided missile with what amounts to a jet fuel warhead. Loading a train or bus up with enough explosives to have an equivalent effect is going to be a really noticeable operation ...