Revenue service tests of tank cars conducted by the Association of American Railroads (AAR) as part of the ‘Freight Equipment Environmental Sampling Tests’ (FEEST) program have measured high coupler vertical loads. It is believed that such high vertical loads can cause significant damage to the structural integrity of tank car stubsills, thereby seriously affecting operational safety. Also, it is suspected that coupler height mismatches are a major source of these vertical forces, and thereby contribute to stubsill failures. The focus of this paper is the study of the effect of coupler height mismatches on stub sill integrity. As part of this study a tank car loaded to 266,200 lb. was instrumented and tested, in static conditions and during impact. The static tests consisted of a series of longitudinal load (squeeze) tests conducted at three different coupler heights, and vertical load tests with forces being applied at the coupler shank. The impact tests consisted of a series of impacts at speeds ranging from 2 mph to 8.5 mph for the following coupler mismatch levels: a) Struck and striking car at the same level, b) Struck car 2″ higher, and c) Struck car 2″ lower. In addition to the testing, finite element models were developed to help in studying additional operating conditions. These models were validated using the test results. The results from the testing indicate that coupler height mismatches have a significant effect on the vertical force levels and the stress levels seen at critical car locations. The stresses in critical areas are generally higher when the struck car is lower. Differences over 50% (compared to when the cars are level) were noted at the head brace, when the struck car was 2″ lower than the hammer car. The tests also established the criticality of vertical forces on the structural integrity of the stubsill. On average, a 50,000 lb. vertical force can generate as much stress as a 680,000 lb. longitudinal force. This work is being funded by the Office of Research & Development of the Federal Railroad Administration (FRA).