The transportation of hazardous materials involves a non-negligible quantity of goods carried by both road and rail. Moreover, considering today's greater environmental awareness and the large anthroprical presence, the release of hazardous materials in an accident assumes an ever growing importance. For this reason a risk assessment, considered as the product of the probability of an incident occurring and the gravity of its consequences, is necessary. In the present work, a model to determine risk levels generated by an accident involving the transportation of hazardous materials by rail is presented. The accident risk assessment model takes into account the influence of (1) the main causes of defects (track structure, line characteristics, wagon characteristics, railway operation and traffic system), (2) the probability of a container breaking with the consequent release of dangerous substances during transport and (3) the number of people potentially exposed to the consequences of the accident. Finally, the presentation of a case study implemented using a Geographical Information System made it possible to highlight the potential and effectiveness of the proposed procedure. It is able to supply information and quantitative data which can be used to characterise improvement works and/or transportation procedures and routes in order to minimise the risks connected to the transportation of hazardous materials.