Following a series of bombings in London this week, the U.S. Homeland security department has changed the security alert from "yellow" to "orange" on public transportation systems in and around cities, but officials say there is no information about a specific threat on American soil. However, the over all national terrorism threat level remains at "yellow".

In recent days transportation officials raised the threat level on mass transit systems to "high," boosting security on passenger railways, subways and bus systems in cities around the United States.

In New York City, for the first time police also started a new random bag search policy, with officers checking the bags of every one-in-five or one-in-ten passengers. New York's Police Commissioner Raymond Kelly called the policy a common sense approach to a changed world.

"We're not picking out anyone other than people identified by this numerical formula and obviously people who are carrying backpacks or large containers," said Mr. Kelly.

Outside cities, regional railways have also added security measures. Amtrak spokesman Cliff Black says the railway increased security following the first series of London bomb attacks on July 7, and security has remained at that level. "It means extended police shifts, higher visibility. K-9 bomb sniffing dogs and generally a higher level of vigilance," he said.

In the nation's capital, Washington D.C. subway spokeswoman Taryn McNeil says many officers on the transit system are undercover. But she says even with additional police officers and closed-circuit cameras, subway officials still depend on passengers to keep a watch for anything suspicious.

"Actually, we depend on the public pretty much to be our eyes and ears out there in addition to the extra security we have," she said. "Because they see alot and hopefully they'll continue to call in."

At the Smithsonian metro stop, not far from the White House on Saturday, hundreds of tourists regularly flowed out of the underground station onto the grassy National Mall. Several people said despite the attacks in London, they were not nervous to ride on Washington's subway system.

Lisa Yacomelli of Princeton, New Jersey, who was leading a group of foreign students on a tour of Washington, said she hoped the extra security would be able to prevent a London style attack.

"One of my friends had her teenaged daughters just above the street when the subway blew in London. And you know, there's kids all over the place here," she said. "I would hope it makes a difference, but I don't know. The explosives can be so small."

She also says she isn't planning to stop riding public transportation anytime soon. "No, because I have to get where I'm going, so I get on it. You gotta do what you gotta do."

U.S. transportation officials say the extra security measures will remain in place untill federal officials decide to lower the threat level for mass transit systems.