The enactment of the Hazardous Materials Transportation Uniform Standards Act of 1990 placed added emphasis on the need to assess the risks and benefits associated with the transportation of hazardous materials by all modes. Rail transportation is playing an increasing role in the growing movement of hazardous materials. The events of September 11, 2001 and the real possibility of future attacks have raised the concern level for rail transportation of hazardous materials and the safety of people and property in the event of a terrorist rail incident. Add to this the ever present possibility of an unrelated rail accident involving hazardous materials especially given the growing volume of hazardous materials.

This paper presents a preliminary risk probability model of a train accident or terrorist incident involving hazardous materials by identifying relevant variables and their applicability to rail movement risk analysis. Although the risk of release due to a rail accident or incident is estimated to be small, it is not impossible. Risk of a low-probability high-consequence accident involving a significant release of hazardous materials must be given adequate consideration.

A delineation of a concept of risk assessment and linking that concept to rail transportation quantitative risk analysis is the initial basis for this paper. It is followed by a review analysis of relevant hazardous materials transportation risk models. Based on these existing models, the paper outlines the elements of a risk based modeling analysis to include types and causes of rail accidents. The paper then defines an initial methodology for carrying out a risk assessment of rail transportation of hazardous materials to include risk of terrorist attack. To accomplish any risk assessment of hazardous substances movement during rail transportation, one must consider the complexity and magnitude of chemicals moving through the rail network. The multiplicity of chemical and physical characteristics of substances, location of manufacturing facility in relation to final destination and volume of cargo tend to further enhance the dimensions and complicate the issue.

Railroads are an essential component of our transportation economy. Some 40% of all intercity freight goes by rail, including 67% of the coal used by electric utilities to produce power and 20% of the chemicals (USDOT, 2005; AAR, 2005a). There are thousands of chemicals in use today and many more are being introduced all of the time. Railroads carry an even higher percentage of those chemicals essential to the public health and standard of living for the United States. Chlorine, for example, used to purify the nation's water supplies has significant rail movements (AAR, 2005b).

Vast quantities of these hazardous substances are being moved by rail alone. Due to the nature of most chemicals, they can pose hazards of explosion, toxic release, and fire. The transportation of these hazardous materials is an important problem due to their pervasiveness. Hazardous materials, or dangerous goods, include explosives, gases, flammable liquids and solids, oxidizing substances, poisonous and infectious substances, radioactive materials, corrosive substances, and hazardous wastes. The fact that the volume of hazardous materials moving by rail more than doubled since 1980 indicates that rail has become an integral part of the tremendous increase in the transport of hazardous materials. Nearly 155 million tons of chemicals are being transported by rail in North America each year which constitutes 1.75 million rail cars of hazardous materials (D'Amico, 2001).

The risk associated with a release of these materials during transportation is what differentiates rail shipments of hazardous materials from rail shipments of other materials. Hazardous materials can be extremely harmful to the environment and to human health since exposure to their toxic chemical ingredients could lead to the injury or death of humans, plants, and animals. The events which have traditionally given rise to hazard during rail transportation are container failure or the impact due to collisions. However, on October 24, 2002 the FBI issued a warning, based on information obtained from al-Qaida prisoners, which said terrorists may try to destroy bridges or key sections of railroad infrastructure and tracks. In its warning, the FBI said captured al-Qaida photographs of U.S. railroad engines, cars and crossings increased the agency's concern about the threat. The 140,000 mile network of rail tracks, bridges, and terminals, more than three times the Interstate Highway System, presents a huge security challenge. Much of the network is in isolated areas where fencing is neither practicable nor effective. Intelligence officials continue to believe that aI-Qaida plans to attack targets such as railroads that would be readily recognized as representing U.S. economic interests. In sum, security has placed a new dimension of concern on the rail system across the country. Terrorism must now be added to the risk equation.