The Federal Railroad Administration sponsored a 2-year research effort between August 2020 and August 2022 to investigate multiple mitigation concepts for hazmat tank cars, aimed at improving their puncture resistance. The team first developed and validated a finite element (FE) model of a full-scale tank car subject to shell (i.e., side) impact. Researchers then evaluated the effectiveness of different mitigation solutions by exercising the validated FE model. Validated FE models were used to explore different solutions including key material properties of tank car steel, the effects of changing shell thickness, the effects of splitting a single thick plate into multiple plates, and internal liner solutions. Results indicated the key parameter to improving puncture resistance is to increase total steel thickness; however, a double-tank arrangement offers improved benefit over an equivalent monolithic shell because of the high strength or high ductility steel used for the outer layer. Moreover, FE analyses indicated the ductility of steel in plane strain bending reduces with increasing plate thickness; the resistance of thick shells may be over-estimated if this is not considered.