Metro began randomly searching passenger's bags for the first time Tuesday. Searches took place at the College Park Metro station in Maryland and the Braddock Road Metro station in Virginia.
The searches began just a day after civil liberties groups launched an online petition opposing the program.
Screeners swabbed some riders' bags and inspected them in a process that took less than a minute. Metro Transit Police say they inspected 25 bags in an hour at College Park.
Metro's program is based on similar efforts in Boston and New York. Metro police say the inspections have been in the works for years and are not a response to any particular threat. However, in recent months there have been various threats to the system.
Metro says the searches will continue at multiple locations throughout the day.