Transportation consultants say mass transit systems make inviting targets for terrorists because they are difficult to secure. They can't effectively use the kind of screening system in place at airports because it would defeat the purpose of transit: moving large numbers of people quickly. People at risk of kidnapping are often told to vary their routines. But mass transit systems are valuable precisely because their routines don't vary. The trains and buses go the same place at the same time every day. That also makes it easier for attackers to study their vulnerabilities. "It's very, very easy to do reconnaissance," says James Carafano, a homeland security expert at the Heritage Foundation.