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(The Associated Press circulated the following article on July 8.)

WASHINGTON — Even in an age of high-tech security equipment, local transit authorities are mainly relying on people and dogs to keep their systems safe from terrorist attack.

Soon after Thursday’s bombings of London’s transit system, police officers in Washington were seen toting MP-5 machine guns on subway trains. State police rode the rails in New York City, and Coast Guard boats escorted the Staten Island Ferry across the harbor.

Extra sheriff’s deputies patrolled Los Angeles Metrolink trains, which mayor Antonio Villaraigosa rode from downtown to Hollywood to reassure passengers that officials were doing everything possible to make the system safe.

On the surface, at least, it was an old-fashioned show of force. Homeland Security Chief Michael Chertoff on Friday said that was appropriate, given that U.S. officials have no intelligence indicating a similar attack is being planned here.

”We thought it was prudent to raise the level in a modest way,” Chertoff said on ABC’s ”Good Morning America.”

Security experts said even higher-tech security is difficult for systems intended to move large numbers of people quickly.

”Very little technology can be applied in this area in an effective way,” said Rafi Ron, former head of security at Israel’s Ben Gurion Airport and now a security consultant in Washington.

The rush-hour London bombings prompted the United States to put its subways, buses and commuter trains on high alert, moving up one notch to the second-highest level, code orange, for mass transit. Planes were not included.

The Homeland Security Department said it would issue a bulletin to federal, state, local and private sector officials reminding them that the rail system remains an attractive al-Qaida target.

Chertoff said authorities had no evidence of a ”specific, credible threat” against the United States. However, he said, ”we feel that, at least in the short term, we should raise the level here because, obviously, we’re concerned about the possibility of a copycat attack.”

The impact of the London attacks was felt all across America.

In Denver, Dave Raven said he had been planning to take a bus on Thursday, but changed his mind and climbed into his car for the commute from nearby Boulder after hearing about the bombings.

”It shakes me up a little bit. I haven’t been using public transportation, and I might avoid it for a while,” said Raven, a 20-year-old University of Michigan student who is living in Colorado for the summer.

Transit authorities nationwide responded to the code orange according to security plans that long had been in place.

The plans, required by the federal government, differed from city to city, but generally called for more police patrols, bomb-sniffing dogs and frequent reminders to passengers to look out for unattended packages.

U.S. counterterror officials said they received intelligence last month dating back to 2004 that al-Qaida terrorists were interested in attacking rail systems in Europe and the United States, including derailing trains or crashing trucks into them. The officials, who spoke on condition of anonymity because the intelligence is classified, said the report lacked specifics on the date and location of any potential attacks.

But there isn’t any high-tech solution on the horizon to change that. And the reason is simple: Transit systems exist to move large numbers of people quickly over a large metropolitan area. Anything that slows the process, such as security checks, would severely disrupt the system.

The Homeland Security Department has looked at high-tech ways to improve transit security but hasn’t instituted anything since testing a system last year.

Following the March 11, 2004, train bombing in Madrid that killed 191 people, the Transportation Security Administration tested a high-tech security system at a suburban Maryland commuter train stop. Walkthrough portals ”sniffed” the air around passengers to check for explosives residue.

The agency said the tests went well but none of the machines have been installed at train stops. They are being used at some airports.

Homeland Security Department spokesman Russ Knocke said the pilot project showed such technology could be used if a specific threat were directed at a specific transit system.

The TSA also experimented with screening checked baggage on long-distance Amtrak trains leaving Washington’s Union Station. In another test, the TSA screened passengers as they boarded trains.

Amtrak spokeswoman Marcy Golgoski said there aren’t any TSA programs to screen people or bags at the passenger railroad stations.

About 29 million people in the United States take commuter trains or subways on an average workday, and millions more take buses. The New York City area accounts for about a third of the rail total, followed by Chicago, Washington, Boston and Philadelphia. The West Coast’s largest transit system is in San Francisco.