No decision has been made on the idea for the city's 106-mile Metrorail system, and the logistics would be difficult. But “it would be another tool in our security toolbox,” says Washington Metropolitan Area Transit Authority spokeswoman Lisa Farbstein.

The possibility is one of many ideas being floated here and elsewhere while the terrorist threat level for transit systems remains at “high” after the July 7 terrorist suicide bombings in London's underground rail tunnels.

Many of the USA's commuter rail and subway systems are much more difficult to secure than airports because they are vast and open. Several cities have bolstered security by adding to what's already available: more cameras, more bomb-sniffing dogs and more announcements reminding people to report suspicious behavior and packages.

Last year, after terrorists bombed rush-hour commuter trains in Madrid, the Homeland Security Department tested explosives detection equipment on some rail passengers at stations in Maryland and Washington, D.C. But because subway systems have so many entrances and exits, it would be impossible to deploy and staff enough of the machines to secure the system.

Some transit systems are looking at random searches and increased inspections by bomb-sniffing dogs.

In Boston, which has the oldest subway system in the country, police officers and dogs are conducting random checks on commuter trains heading into the city center, Massachusetts Bay Transportation Authority spokesman Joe Pesaturo says. The canine teams are patrolling the trains but are “not asking anyone to open their bags,” he says.

Last summer, when the Democratic National Convention came to town, Boston security chiefs approved random passenger checks for several weeks using explosives-detection swabs — high-tech equipment that must be run over the outside of a bag. Officers also stopped all trains before they went under the convention center and checked passengers' bags.

The American Civil Liberties Union had objected to random searches at all stations — and likely would do the same in Washington.

John Reinstein, legal director of the ACLU of Massachusetts, says there's no fair way for transit systems to conduct random searches.

“The suggestion that we're just going to do it randomly literally invites abuse and focusing on people who for subjective and unverifiable reasons are more suspect, which means that their skin is a different color,” he says.

In New York City, where police have dramatically stepped up patrols in the subway since the London bombings, Metropolitan Transportation Authority spokesman Tom Kelly says there are no plans to conduct random searches.

If someone is acting suspicious or a bag is left unattended, the system would take a closer look. “But do we just stop people walking through the door? The answer is no,” Kelly says.