U.S. communities depend on reliable, safe, and secure rail systems. Each weekday, more than 12 million passengers take to U.S. railways. Recent attacks on passenger-rail systems around the world highlight the vulnerability of rail travel and the importance of rail security for these passengers. The use of passenger rail and the frequency with which terrorists target it call for a commitment to analyzing and improving rail security in the United States. This book explains a framework for security planners and policymakers to use to guide cost-effective rail-security planning, specifically for the risk of terrorism. Risk is a function of threat (presence of terrorists with intent, weapons, and capability to attack), vulnerability (likelihood of damage at a target, given an attack), and consequences (nature and scale of damages if an attack succeeds). While effective security solutions may address all three components of risk, this book focuses on addressing vulnerabilities and limiting consequences, since these are the two components of risk most within the realm of rail-security personnel. The analysis is based on a notional rail system that characterizes rail systems typically found in the United States. The methodology presented is useful for planning rail-security options.