Columbia subject headings:
Bus lines--Security measures--United States.
Local transit--Security measures--United States.
Railroads--Passenger traffic--Security measures--United States.
Railroads--Security measures--United States.
Transportation--Security measures--United States.
Harvard subject headings:
Bus lines--Security measures--United States.
Local transit--Security measures--United States.
Railroads--Passenger traffic--Security measures--United States.
Railroads--Security measures--United States.
Transportation--Security measures--United States
Princeton subject headings:
Bus lines--Security measures--United States.
Local transit--Security measures--United States.
Railroads--Passenger traffic--Security measures--United States.
Railroads--Security measures--United States.
Transportation--Security measures--United States.