From police with machine guns at New York's Grand Central Terminal to extra identity checks at random Amtrak stations, security at the nation's subways and railways was stepped up almost immediately after the London bombings.
While intelligence and law-enforcement officials stress there has been no threat against the rail system in the United States, they're also cognizant that the nation remains a target. Even more worrisome, the apparently coordinated wave of bombings in London is raising new questions about the gaps in security that remain in the US transportation system - and, by most accounts, they are considerable.
"Obviously we're concerned about the possibility of a copycat attack," said Homeland Security Secretary Michael Chertoff. "[But] I think our transit systems are safe."
Since 9/11, security has been tightened on Amtrak and in subways in the form of increased police presence, random searches by bomb-sniffing dogs, and significantly boosted security training for personnel. In most major train stations, trash cans are now bomb-resistant, and agents specially trained in biohazards and explosives routinely patrol.
Homeland Security officials have also undertaken several pilot projects, including experimenting with sensors to detect weapons of mass destruction and creating special vehicles for security screening.
And in the past two years, the Transportation Security Administration has devoted $10 million to mobile canine explosive inspection teams that go to key facilities like train stations, terminals, and passenger rail cars, according to Russ Knocke, a Department of Homeland Security spokesman. The department has also allocated $115 million to help urban areas increase security of mass transit. And immediately after the Madrid bombings, it hired 100 new rail security inspectors.
"We're always looking for ways to enhance our rail-security capabilities, information-sharing, and advanced technology," says Mr. Knocke.
Still, critics have long contended that rail security is being shortchanged because of the emphasis on airline security. They're additionally frustrated because the very nature of rail and subway service makes it difficult to protect, as the recent bombings of mass-transit commuter systems in London, Madrid, and Moscow have shown.
The biggest problem is their sprawling nature. Train tracks run for millions of miles throughout the US, with thousands of bridges and tunnels in densely populated urban areas and remote mountain locations.
Most major train stations have numerous entrances and multiple tracks. And along with the subways, millions of people ride the rails everyday.
"There are essentially no security arrangements for commuter rail. It's a very difficult problem," says Steven Simon, a senior analyst at the RAND Corp. in Washington. "Ridership is just huge, the density of the users is very high, and people are in a hurry. It hasn't proved practical yet to search people or pass their bags through any kind of screening device."
As a result, the best defense, say analysts, is a combination of good intelligence, highly trained crews, and an alert ridership. Across the country Thursday, the heightened vigilance was clear - especially when the terror alert was raised to code orange for mass transit.
In New York's Grand Central Terminal, commuters were greeted with the sight of blue-clad police officers armed with automatic weapons. The police were also much more noticeable in the subway system.
Andy Lachman, traveling on the Times Square Shuttle, says the explosions in London "are in the back of my mind." It prompted him to be "more vigilant," but, he adds, "it's really out of your control."
In Pennsylvania Station, there were continuous loud-speaker announcements for passengers to guard their luggage. National Guard troops mingled with transit police. The increased security met with the approval of Shirley Juste, who was on her way to Richmond. "Better safe than sorry," said Ms. Juste, as security police eyed the line to get on the train.
From Boston to Chicago to Los Angeles, security was similarly stepped up. "We're definitely increasing police presence. We've tightened security measures. We are at an elevated state," says Lydia Rivera, of the Massachusetts Bay Transportation Authority in Boston. "But overall, we are just going to run business as usual."
In Chicago, police are also increasing their presence in train stations and bringing in bomb-sniffing dogs. Frank Kruesi, president of the Chicago Transit Authority, said at a press conference Thursday, "We have people trained to pay attention - employees and increasingly customers."
Just that morning, he said, someone reported a suspicious package on the Brown Line. It turned out to be something a rider had simply forgotten and came back for, but that kind of vigilance is what's necessary, he said.
In Washington, New York, and around the world, intelligence gathering is also being stepped up.
"The best defense is good information and good intelligence and prophylactics that can be put in place based on that," says Frank Cilluffo, director of the Homeland Security Policy Institute at George Washington University in Washington. "I can assure you that every bush is being shaken right now to see what comes out."
But analysts also caution that in this war on terror, where the combatants are more ideologically than geographically based and sometimes instigate terror on their own, gathering that intelligence remains one of the greatest challenges.
"Intelligence would be a great remedy. But the whole point here is that these groups are coming out of the woodwork, and it's not clear how law enforcement or intelligence agencies will get timely and detailed information on groups that seem to form spontaneously," says Mr. Simon. "The answer probably lies in a mix of rider vigilance and increased law-enforcement presence."
• Ron Scherer and Anna Levine-Gronningsate in New York, Amanda Paulson in Chicago, and Alexander Dworkowitz and Adam Karlin in Boston contributed to this report.