The U.S. Transportation Security Administration (TSA) plans to deploy a number of federal air marshals to the U.S. mass transit system as part of a program to evaluate the merits of increased security on trains, subways and other public transportation systems. Although federal attention to mass transit security is long overdue, the measures unlikely would prevent a well-planned attack. Air marshals, who normally provide security on airliners, will be stationed along commuter rail lines in the northeast and in Los Angeles, as well as on ferries in Washington state, and mass transit systems in Atlanta, Baltimore, Washington, D.C., and Philadelphia. The air marshals will be deployed in "Viper Teams" consisting of two marshals, TSA inspectors and bomb-sniffing dogs, and at least one local law enforcement officer. Some team members will be in uniform and easily identifiable as TSA personnel, while others will work undercover, as do marshals on airline flights. Unlike an airliner, which becomes a sealed system once in flight, however, a commuter train is in a constant state of flux, as passengers board and depart. Also, although it is possible to pass between cars on Amtrak trains, this type of movement is not possible in many subway systems — meaning marshals would be trapped in one car until they reach the next station and could not intervene should an attack occur in another car. Furthermore, the trains that would present the most tempting targets for a mass casualty attack — rush hour subways in Washington and New York — also pose the largest challenge to secure. Unlike aircraft, where passengers are seated and aisles are kept clear, subway passengers often stand and are packed tightly into the cars. Because of this, it will be hard for marshals to see the majority of people in the train car in which they are riding, or to take action if they do encounter a threat. The marshals will need to examine and alter their tactics and procedures to account for this very different environment. On the other hand, the countersurveillance teams to be posted on platforms and in other parts of the stations will be more effective. They will be in position to make long-term observations of people in the station, which will enable them to spot potential terrorists during the pre-operational planning stage of the attack cycle. Also, the visible presence of security personnel could be seen as one more layer of defense, and thus deter some less-determined potential attackers. When presented with the option of attacking a target with a visible security presence or a target without one, the latter often is preferable. This is good for the larger stations that will have visible security — though any terrorist could easily attack or enter the system at one of the smaller stations. The July 7 and July 21 London Underground bombers entered the system at minor stations — and the July 21 bombers attempted their attack even after security was increased following the first attack. Given the fact that local law enforcement historically has been responsible for providing security on metropolitan mass transit systems, this new TSA plan represents a major shift in the federal government's homeland security policy. The TSA likely will discover, however, that the job is just too big.