Security experts and members of Congress urged the Bush administration yesterday to step up long-term security efforts to protect the nation's mass-transit riders after the deadly explosions that struck London's subway and bus system.

Homeland Security Secretary Michael Chertoff raised the threat level to orange, or high risk, for mass transit yesterday, enacting a series of short-term protective measures, including more police officers, extra barriers, increased video surveillance, and added inspections of trash bins and other potential hiding places for bombs. In addition, he urged transit authorities to increase inspections of passengers and their bags in some areas.

There is no "specific, credible evidence of an attack that's imminent in the United States," Chertoff said in Washington yesterday. "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."

At the same time, Chertoff urged Americans to continue using public transportation. "This is not an occasion for undue anxiety," he said.

Since terrorists bombed trains in Madrid on March 11, 2004, the Transportation Security Administration has taken some steps to improve security for mass transit, but its budget for airline security dwarfs that of rail, subway and other ground efforts. Officials have conducted vulnerability assessments of rail systems, hired rail inspectors and run tests of explosives-detection technology at some locations -- such as the Metro station in New Carrollton. But security experts said the efforts have resulted in few substantive improvements.

"Madrid was a big wake-up call to us, yet we did relatively little," said Clark Kent Ervin, the former Homeland Security inspector general who is now at the Aspen Institute. "It really seems to me a matter of time before it happens in our country -- in part because it's so easy to do."

From 1998 to 2003, 181 attacks have occurred on transit systems worldwide, resulting in 431 deaths and several thousand injuries, according to Rand Corp., an independent research group. Most were carried out by separatist groups.

The FBI and other U.S. intelligence agencies did not detect any signs of an attack before the bombings in London yesterday, but U.S. authorities are reviewing recent reports to make sure no clues were missed, officials said.

A senior administration official acknowledged the difficulty U.S. authorities have had in trying to figure out how to protect the public transportation systems against attacks. He said that because of the sharp focus on aviation security, terrorists are looking for new targets. "Public transportation remains very vulnerable. We've known this -- we knew this even before Madrid," said the official, who works on U.S. defense policy.

Experts said transit systems, particularly subways that move rapidly through multiple stations, remain attractive to terrorists because it is easy for them to plant explosives and escape.

"The notion that you can, with 100 percent certainty, prevent this kind of incident from happening in the U.S. or anywhere else is absurd," said K. Jack Riley, a Rand Corp. transportation security expert. "The unsatisfying answer to the public in how to prevent these incidents stretches back to having a comprehensive strategy to reduce the appeal to this radical ideology and efforts to disrupt and demobilize" terrorist groups, he said.

The move to code orange marks the first time the terrorist threat level has been raised since August 2004, when Homeland Security officials increased the level for financial sectors in three northeastern cities because of evidence that terrorist operatives had cased buildings before the Sept. 11, 2001, attacks.

Many of the nation's 14 million daily mass-transit commuters noticed new security measures yesterday morning on their way to work before the announcement. At 6 a.m., New York City was one of the first to begin adding police officers to transit systems, and city officials reported no noticeable decrease in ridership. The police department doubled its normal number of 3,000 officers assigned to the mass-transit system and put an officer on every train during the morning commute. City and state agencies are monitoring the city's water supply and air for biological and chemical weapons.

Some wary New Yorkers said the London bombings had them rethinking their subway routes and plans for the day. "I didn't want to go to work anymore, but I had to take a chance," said accountant Michael Johnson, 41, on his way to lunch. "I tried to leave home a little earlier and miss the morning rush."

On Boston's T subway system, a message repeatedly asked passengers to be on the lookout for suspicious activities "now more than ever" and gave a phone number to call to report anything out of the ordinary. "If you see something, say something," the message said.

Passengers became confused at midday, when Boston's green line, one of the city's busiest public transit routes, was shut down because of a minor collision between two trains that injured at least one person. Commuters streamed out of the subway and onto shuttle buses, escorted by Massachusetts Bay Transportation Authority officials and state police. Many stopped to ask if the closure of the line was related to the attacks in London. "Just an unfortunate coincidence," a policeman said.

In Chicago, officials monitored the 2,000 surveillance cameras stationed across the city. Police with bomb-sniffing dogs patrolled the El train's orange line, where riders said they had no choice but to use the system to get to work and other activities.

"I'm sure people are concerned it can happen here, you never know," said former Marine Hank Lemecha, 58, in Chicago. But he added: "I'm more worried about the train falling off the track since we're 110 feet up."

Amtrak said it stepped up security across its national system yesterday, including deploying bomb-sniffing dogs at rail stations and adding more law enforcement officers to patrol the areas. Since the Madrid bombings, Amtrak now requires identification for people buying train tickets at the counter, and employees check IDs randomly as passengers board trains, a spokeswoman said.

Riley and other security experts said the federal government could do more to improve mass-transit security by increasing camera surveillance, inspecting trash bins more thoroughly and expanding training of employees and passengers to report suspicious packages.

The American Public Transportation Association said its members, who operate municipal transit systems in U.S. cities, need $6 billion in security improvements, with a priority on expanding the ranks of police officers. "The reality is the federal government's funding is woefully inadequate," said Rose Sheridan, spokeswoman for the group. "Since 9/11, the aviation industry received $18.1 billion. The public transit agencies have received $250 million and there's 16 times more riders on public transit."

On Capitol Hill yesterday, Democrats and some Republicans vowed to add millions of dollars to the Homeland Security Department's 2006 budget for rail, bus and other transit security, even though similar efforts have not passed in previous years. Homeland Security missed a congressional deadline earlier this year to provide a strategic plan for how to protect the nation's transportation systems. The plan was to be used as a guide for spending.

"We're going to have to refine what it is we're trying to do and TSA has just not done that. What do we do to screen people on mass transit? How can we do it?" said Rep. Harold Rogers (R-Ky.), chairman of the House Appropriations homeland security subcommittee. "In spite of everyone's pleading with them, we haven't been able to get TSA to focus on mass-transit security."

Staff writers Peter Slevin and Kari Lydersen in Chicago, Jonathan Finer in Boston, and Michelle Garcia in New York contributed to this report.

Members of the New York Police Department's emergency services unit patrol at Grand Central Terminal after terrorist bombs went off on subway trains and a bus in London. The Department of Homeland Security immediately increased protective measures for mass transit.