Federal law enforcement officials tell ABC News a plot designed to use 15 to 20 suicide bombers on one commuter train as close to Sept. 11 as possible was well underway. The specific target was the PATH commuter trains that run in a tunnel under the Hudson River into New York City. "This is a plot that would have involved martyrdom, explosives and certain of the tubes that connect New Jersey with lower Manhattan," said Mark Mershon, Assistant Director-in-Charge of the FBI New York Field Office. "We're not discussing the modality behind, beyond that." But law enforcement officials say the...