FRA sponsored research to evaluate the side impact strength of Tier I passenger rail equipment designs built to the current rollover and side structure regulations. This report presents results from the first stage of this research program, including: (1) the makeup of the U.S. passenger car fleet; (2) the accident history involving side structures in the U.S.; (3) a review of passenger rail-side structure design and performance criteria in current standards and regulations; (4) an evaluation of proof loads for similar vehicle structures used in other modes of transportation; and (5) a discussion of the technical challenges in proposing side impact criteria.