The surface transportation system is vital to our nation's economy, defense, and quality of life. Because threats against the system have hitherto been perceived as minor, little attention has been paid to its security. But the world is changing, as highlighted by dramatic incidents such as the terrorist chemical attack on the Tokyo subway in 1995. As a consequence, security concerns are now attracting more attention--appropriately so, for the threat is real, and responding to it is hard. Although the surface transportation system is remarkably resilient, it is also open and decentralized, mak...