Within hours of the London bombings, a renewed call went up for the United States to use its considerable technology heft to prevent similar attacks on the nation's transit system.

Public transit's chief lobbyist said its members need $6 billion to upgrade security, and Congress is expected to increase funding in the coming weeks. Sensing opportunity, some technology companies aggressively advertised their potential to create gadgets to detect bombs and chemical and biological weapons.

But ideas such as smoke-detectorlike devices sounding an alarm when a bomb-porting terrorist enters a train station are years and billions of dollars from fruition — if ever. The best current defenses for the country's subways, buses and trains, security experts say, remain decidedly low-tech: human vigilance and bomb-sniffing dogs.

The very nature of mass transportation makes it impossible to install metal detectors and take the other security measures that aim to protect the flying public.

"You cannot just take the applications that are used in airport and plunk them into the transit system," said Gregory Hull, director of safety and security programs for the American Public Transportation Association. "But some could be modified."

The industry has spent $2 billion since Sept. 11, 2001, training its security personnel to be on the lookout for abandoned packages and suspicious passengers.

Nonetheless, the system needs about $5 billion in radio communications improvements, and the industry is also keen to deploy more cameras to surveil tunnels and stations, he said.

Another promising technology under development is software that would direct cameras to immediately flag suspicious scenes at stations such as abandoned packages or passengers dressed for winter during summer.

Sniffing the air for hints of bombs, though, remains an elusive holy grail.

"Detecting explosives is not an easy thing," said David Danley, a retired Army colonel and head of defense programs at Combimatrix Corp., a small biotechnology company near Seattle.

Combimatrix has $10 million worth of contracts to develop a hand-held device that soldiers can use to detect pathogens on the battlefield. Danley said the device, which is fueled by so-called "gene chips," could be adapted to detect chemicals as well and shrunk to the size of smoke detector.

The detector would contain key genetic signatures of dozens of deadly bugs and chemicals on a chip. If an attack occurred, DNA from the weapon would bind to the chip, which would trigger an alarm.

Danley conceded that in developing a detector, determining exactly which of the myriad deadly bomb-making agents to sniff for is the "$64 million question."

"We won't come up with a 100 percent solution," he said.

Still, since last year's Madrid bombings, the Department of Homeland Security has increased the money available for transit security.

"I can envision technologies in the future that will give us the ability to detect explosives without going through the kind of portals you do in the airport," Homeland Security chief Michael Chertoff said.

Among the slew of companies offering improvements is Irvine, Calif.-based HiEnergy Technologies Inc., which makes detectors that can determine in 15 seconds to three minutes whether an object contains explosives and, if so, which ones. The suitcase-sized detectors, which cost $300,000 apiece, bathe an object in neutrons and measure the gamma rays emitted in response.