BOSTON (CBS) – MBTA police say they’re taking steps to increase security in the wake of a new terror threat.

Documents seized from the bin Laden compound reveal that Al Qaeda considered attacking trains in the United States.

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On Thursday, a Homeland Security intelligence warning sent to law enforcement officials around the country says that as of February 2010, al Qaeda was looking at ways to tamper with train tracks so that a train would derail into a valley or off a bridge.

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The memos reference potential attacks against major American cities – New York, Washington, Chicago and Los Angeles. CBS sources say the writings suggest striking on important specific dates like July Fourth, September 11th and New Year’s.

WBZ-TV’s Jonathan Elias reports

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MBTA police issued a statement Thursday night saying that even though Boston is not specifically mentioned, they are stepping up patrols.

“In addition to our regular Transit Police patrols, we have increased our high visibility patrols on our subway lines and we continue to conduct security inspections at selected stations,” said the statement from MBTA Transit Police Chief Paul MacMillan. “We also continue to work closely with our federal, state and local partners in a cooperative partnership to ensure the timely sharing of information and intelligence that may impact the MBTA.”

MacMillan says MBTA and MBCR employees and passengers should remain vigilant and continue to report any unusual or suspicious incidents.

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According to CBS Sources, the bin Laden memos do not indicate that any of the potential plots are “operational.”