Columbia subject headings:
Bus lines--Security measures--United States.
Buses--Security measures--United States.
Railroads--Security measures--United States.
Terrorism--United States--Prevention.
Transportation--Security measures--United States.
Harvard subject headings:
Bus lines--Security measures--United States.
Buses--Security measures--United States.
Railroads--Security measures--United States.
Terrorism--United States--Prevention
Transportation--Security measures--United States
Princeton subject headings:
Bus lines--Security measures--United States.
Buses--Security measures--United States.
Railroads--Security measures--United States.
Terrorism--United States--Prevention.
Transportation--Security measures--United States.