Details about the plot appeared to be some of the first tangible information about al Qaeda's activities to come from materials confiscated at the compound in Pakistan where Osama bin Laden was killed on Sunday.
DHS issued a message on Thursday to state and local law-enforcement agencies about the plot, but emphasized the planning occurred in February 2010 and, by all accounts, was only conceptual.
"As one option, al Qaeda was looking into trying to tip a train by tampering with the rails so that the train would fall off the track at either a valley or a bridge," according to the message, obtained by National Journal and marked "for official use only."
"Al Qaeda noted that an attack from tilting the train would only succeed one time because the tilting would be spotted," the message added. "While it is clear there was some level of planning, we have no recent information to indicate an active, ongoing plot to target transportation and no information on possible locations or specific targets."
DHS spokesman Matthew Chandler said "it is unclear if any further planning has been conducted since February of last year." He added there is no indication there was anybody inside the United States casing trains or train stations.
The Homeland Security Department is not aware of any credible or specific information that would warrant raising the nation's terrorist threat level, Chandler said. But the department decided to notify state and local officials of the information, given the events of last weekend when bin Laden was killed.
According to the department: "Since Sunday, DHS and its partners have taken a number of actions, including but not limited to: reviewing protective measures for all potential terrorist targets, including critical infrastructure and transportation systems across the country; deploying additional officers to non-secured areas at our nation's airports; and identifying any new targeting rules that should be instituted to strengthen the ways we assess the risk of both passengers and cargo coming to the United States."