The paper first provides background information on foreign-based terrorist attacks in the United States and an overview of the characteristics of U.S. rail systems. The discussion addresses the features of likely terrorist threats to the United States and its rail systems, with attention to vulnerabilities and critical elements of freight railroads and passenger rail systems. Also discussed is the U.S. Defense Department's role in protecting critical rail infrastructure. Recommendations are offered on what features of rail systems should receive the most protection and the characteristics of the protection. The priorities recommended are the protection of transit rail stations in the largest, most densely populated cities with a history of terrorist attacks; rail shipments of hazardous materials through large metropolitan areas; and passenger trains and rail stations for those trains. Three actions are recommended for protecting these rail system assets. First, there should be accelerated development of high-volume, walk-through chemical, biological, and radiation sensors that screen passengers and bags at transit stations. Second, hazardous cargo rail shipments should be rerouted around cities with high-density populations. Third, there should be an integrated National Transportation Strategy that specifies priorities, designs, resources, time tables, and monitoring for improvements in railway security throughout the United States. 7 figures and 96 notes