WASHINGTON -- The very nature of public transit systems presents security planners with an ever-more-delicate balancing act.
As was underscored in last week's terrorist attacks on the London subway and bus systems, such systems' open architecture makes them vulnerable to attack in a way that the more-controlled access to an airport can prevent. Yet tightening security too much would defeat the purpose of public transportation systems, which is to move many people quickly and efficiently.
The London attacks led the U.S. Department of Homeland Security to raise the threat level for mass transit to code orange-high-from the previous code yellow, or elevated, level. The heightened threat level applies only to regional and intercity passenger rail, subways and metropolitan bus systems.
Still, transit systems can bolster their security without disrupting their operations too greatly, say security experts. Increased vigilance, of both the human and electronic varieties, can help deter attack, they say. Dogs, if properly trained, can be a valuable asset as well.
The security challenges presented by transit systems cannot be overestimated, say experts.
"These transit systems are so hard to control because they're very open and require public access," said Christopher Grniet, vp at Kroll Inc.a New York-based unit of Marsh & McLennan Cos Inc. "By their very nature, putting additional security in could make them ineffective," he said.
"It's very difficult to control that environment," said Mr. Grniet. He noted that after last year's train bombings in Madrid and repeated Irish Republican Army bomb attacks in London over the years, transit systems tightened security.
"But to what extent you can do that and still operate those systems to the benefit of the public without impacting our daily lives is questionable," he explained.
"The difficulty with buses and most light rail systems is they stop too many times and the volume is too great," said Neil C. Livingstone, chief executive officer of Global Options Inc., a Washington-based risk management services company.
Hank Chase, a director in the Alexandria, Va., office of Devon, Pa.-based Smart & Associates, a business consulting and accounting firm, called the London attacks "particularly frightening." He noted that the level of security was "especially high" because of the G8 Summit in Scotland. He said that "it is fair to say" that the level of security before the attack "was much higher than in comparable U.S. transportation systems."
"If you look at vulnerability, we are much more vulnerable, on average. A Metro station in D.C. is, on average, more vulnerable than a London underground station. When you deal in threat and criticality, there are all kinds of nuances," he said.
Adding to the potential destruction is the fact that subways operate in "underground, closed contained spaces," he said. A train in a tunnel becomes "essentially, a big pipe bomb."
Despite the magnitude of the threat, transit systems have both limited resources and limited technology with which to respond, say security experts. Yet those in charge of operating such systems must ramp up their efforts.
"For subways, all they can do is increase their patrols, increase their electronic surveillance systems and ask people to be vigilant," said Kroll's Mr. Grniet.
This response was evident in Washington's Metro stations within hours of the London attacks. Metro employees handed out small cards bearing the heading "SEE IT? SAY IT!" The cards urged riders to report unusual behavior, unattended packages or "anything that seems suspicious" to Metro employees or to call Metro Transit Police or 911.
The reverse of the card directed readers to safety-related Web sites for further information.
Other transit systems posted statements on their Web sites calling for vigilance while reassuring riders that security measures were being implemented. For example, Faye Moore, the general manager of the Southeastern Pennsylvania Transportation Authority in Philadelphia, posted a statement saying, "I rode the train this morning, and I will ride it home at the end of the day. I believe the system is safe."
"We ask our passengers to be vigilant for suspicious objects or people," the statement said.
Electronic surveillance is also critical, said Mr. Livingstone. He noted that there are thousands of surveillance cameras in the United Kingdom in transit systems and elsewhere. U.S. transit systems need more cameras, he said.
Mr. Chase called for "deploying armed, sworn officers in front of these stations, ideally with dogs that are trained to smell explosive residue."
"Nothing beats a well-trained police officer, hopefully with a dog, pulling people aside and seeing if their sixth-sense suspicion" is correct, he said.
"We don't have nearly enough bomb sniffing dogs," said Mr. Livingstone.
"If somebody appears suspicious, they've got to grab them," Mr. Chase said.
More-drastic measures may be required to meet security needs, said Mr. Livingstone. "Ultimately, what you may have to do" is limit what people can carry onto mass transit. This might mean banning backpacks, he said. Doing so would "make it that much harder to leave something under the seats," he said.
"The problem is we have maybe 1/100th of the resources that we need," said Mr. Chase.
"But we have to keep in mind that there is no silver bullet to prevent these types of events from happening," said Mr. Grniet. "All we can do is take prudent mitigation measures and to remain vigilant and ask the public to remain vigilant."