Carl Frankel can't help feeling it's just a matter of time before this city, still deeply scarred from the 2001 terrorist attacks (search), will once again be in the crosshairs.

"Just knowing the inevitability of it, it feels depressing and worrisome," said the 61-year-old clothes pattern maker in Manhattan. "I think we are all frustrated that this situation is continuing — now for four years — and we don't know what to do about it."

Similar apprehension was expressed around the country in the aftermath of the London bombings (search), though few said they would alter their daily routines.

Reminders of the potential of terrorism were everywhere. Extra police officers and bomb-sniffing dogs were visible at train stations, bus depots and airports in many big cities.

At Boston's Park Street subway station, an announcement telling people to report suspicious activity played repeatedly over an intercom.

Two subway stations in a suburb of Washington, D.C., closed for about an hour after commuters reported suspicious packages — later determined to be harmless. In San Francisco, the Bay Area Rapid Transit system closed bathrooms in all of its stations as a security precaution.

In New York, vehicles entering bridges, tunnels and airports were subject to random inspections. Guards watched over the city's water supply. Patrol boats escorted ferries to Staten Island.

Soldiers carrying rifles strolled train stations while thousands of police officers finishing their shifts were held over for extra duty. Police said that, until further notice, every subway train in the city would have at least one uniformed officer on board.

Dave Hoops, 47, a pension administrator from Long Island, said he was heartened by the extra security.

"I'm not scared. I'm not frightened," he said. "New York is New York. As you can see, there is plenty of protection."

Mayor Michael Bloomberg (search), Los Angeles Mayor Antonio Villaraigosa and New York Gov. George Pataki all made it a point to ride mass transit Thursday to illustrate their faith in its safety.

Other commuters said they would continue to ride, too, despite fears about security.

"It makes you wonder about the security you have in our transportation system," Paul Stark, 42, of Kildeer, Ill., said of the overseas attack as he rode a train to Chicago. "But at the same time you can't manage your life around events that you don't have control over."

William Orem, 39, an editor for a Boston publishing company, said he felt detached from the events in London — a feeling he acknowledged he shouldn't have.

"I've got that strange suspended feeling that a lot of Americans have, that until it's going on (here), until someone starts blowing up my subway, it still feels like it's somewhere else in the world," he said.