Improving the design of higher-capacity railway tank cars for hazardous materials transport: optimizing the trade-off between weight and safety.

As with many aspects of modern industrial society, decision-makers face trade-offs in considering hazardous materials transportation equipment and practices. Tank cars used for transport of hazardous materials can be made more resistant to damage in accidents through use of a thicker steel tank and other protective features. However, the additional weight of these features reduces the car's capacity and thus its efficiency as a transportation vehicle. In this paper the problem of tank car safety versus weight is developed as a multi-attribute decision problem. North American railroads recently developed specifications for higher capacity tank cars for transportation of hazardous materials including enhanced safety design features. A group of tank car safety design features or "risk reduction options" (RROs) were analyzed with regard to their effect on the conditional probability of release in an accident, and their incremental effect on tank car weight. All possible combinations of these RROs were then analyzed in terms of the reduced release probability per unit of weight increase and the Pareto optimal set of options identified. This set included the combinations of RROs that provided the greatest improvement in safety with the least amount of additional weight for any desired level of tank car weight increase. The analysis was conducted for both non-insulated and insulated tank cars and used two objective functions, minimization of conditional probability of release, and minimization of expected quantity lost, given that a car was derailed in an accident. Sensitivity analyses of the effect of tank car size and use of different objective functions were conducted and the optimality results were found to be robust. The results of this analysis were used by the Association of American Railroads Tank Car Committee to develop new specifications for higher capacity non-insulated and insulated, non-pressure tank cars resulting in an estimated 32% and 24% respective improvement in safety.