All transit systems, not just subways.
From WABC - NY
Feds warn of terror plotting against NYC subways
Federal authorities are warning law enforcement personnel of a possible terror plot against the New York City subway system during the holiday season.
An internal memo obtained by The Associated Press says the FBI has received a “plausible but unsubstantiated” report that al-Qaida terrorists in late September may have discussed attacking the subway system.
Anti-terror agencies say they have no specific details to confirm the plot has moved “beyond aspirational planning,” but are issuing the alert out of concern that an attack could come during the holiday season,” warns the memo, which is dated Tuesday.
While federal agencies often issue all sorts of advisory warnings, the language of this one is particularly forceful.