As of February 2010, Al-Qai'da "was allegedly contemplating conducting an operation against trains at an unspecified location in the United States on the 10th anniversary of September 11, 2001," the US Department of Homeland Security (DHS) advised law enforcement agencies in a bulletin obtained by AFP.
A source said the warning arose from intelligence seized in a daring raid on bin Laden's fortified compound in which elite US commandos shot dead the terrorist mastermind and gathered computer hardware and other material.
The official message noted that it was based on "initial reporting" and warned that such information "is often misleading or inaccurate due to a rapidly developing situation and is subject to change."
"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," the bulletin said.
Al-Qai'da "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 department.
The terror network "noted that an attack from tilting the train would only succeed one time because the tilting would be spotted."
Al-Qai'da "also noted that newer train cars each have their own braking system, and that movement in a specific direction would derail it, but would not cause it to fall off the track," it said.
The US Transportation Security Administration (TSA) was to warn "rail sector stakeholders" soon, the bulletin said.
A DHS spokesman, Matthew Chandler, emphasized in a statement that the department had "no information of any imminent terrorist threat to the US rail sector."
"It is unclear if any further planning has been conducted since February of last year," Chandler said, adding that the US government was "at a heightened state of vigilance" but would not issue a new terrorism alert.
But Chandler also said that the department and its partners in US law enforcement had taken protective steps since the Sunday raid that left the world's most hunted man dead and a treasure trove of his secrets in US hands.
Officials have been reviewing protective measures for all potential terrorist targets, including critical infrastructure and transportation systems nationwide, deploying additional officers to public areas of US airports, and looking for ways to improve the security of air passengers and cargo, he said.
The new warning, which had the effect of emphasizing that bin Laden seemed to be still integral to Al-Qai'da's operational planning as of February 2010, came as US officials pored over the fugitive's seized files.
Experts believed the material -- including about five computers, 10 hard drives and 100 storage devices -- could prove invaluable to deal further blows to Al-Qai'da 10 years after the deadly attacks on New York and the Pentagon.
US officials said a task force of experts from the Central Intelligence Agency, the National Security Agency, the Justice Department and other agencies had begun scouring the material, in a task that would last months and possibly years.
The initial search will focus on "detecting ongoing threats" and information pointing to "other high-value targets within Al-Qaeda," Michael Leiter, head of the National Counter-terrorism Center, told National Public Radio.
The government would "probably" be adding names to its terrorist watch list as a result of the information gleaned from Bin Laden's computer files, Attorney General Eric Holder told a Senate hearing Wednesday.
Cyber specialists will first dismantle the hardware, taking care for possible booby traps or triggers that would erase files. Then they will extract and copy all the stored data, sifting through temporary files and trying to crack encryption codes, analysts said.
