New York City is beefing up security on its public transportation systems ....The F-B-I warning of a possible holiday terror plot against the city's subway and train lines.An internal F-B-I memo obtained by the associated press calls the threat from Al Qaeda quote plausible but unsubstantiated ...Homeland Security leaders confirm a warning is out there, saying Al-Qaeda may have discussed targeting the transit system as far back as September.While the plot is specific to New York City, commuters across the country are urged to take precautions.