"The nation's air, land, and marine transportation systems are designed for accessibility and efficiency, two characteristics that make them highly vulnerable to terrorist attack. While hardening the transportation sector from terrorist attack is difficult, measures can be taken to deter terrorists. The dilemma facing Congress is how best to construct and finance a system of deterrence, protection, and response that effectively reduces the possibility and consequences of another terrorist attack without unduly interfering with travel, commerce, and civil liberties. […] Bombings of passenger trains in Europe and Asia in the past few years illustrate the vulnerability of passenger rail systems to terrorist attacks. Passenger rail systems--primarily subway systems--in the United States carry about five times as many passengers each day as do airlines, over many thousands of miles of track, serving stations that are designed primarily for easy access. Transit security issues of recent interest to Congress include the quality of TSA's surface transportation inspector program and the slow rate at which transit and rail security grants have been expended. Existing law mandates the scanning of all U.S.-bound maritime containers with non-intrusive inspection equipment at overseas ports of loading by July 2012. This deadline was not met, in part because foreign countries object to the costs of this screening and are dubious of the benefits. Implementation of the Transportation Worker Identification Credential (TWIC) for port and maritime workers also appears to be experiencing continuing difficulties."
CRS Report for Congress, RL33512