The mainline freight train derailment rate of major railroads in the United States declined 49% from 2006 to 2015. Nevertheless, derailments remain the leading cause of major railroad accidents. Identification and quantification of the types of train accidents, the trackage where they occur, and the causes having the greatest effect on train safety and risk is critical to determine the most effective strategies for further improvement. Federal Railroad Administration data were used to quantify factors contributing to the declining accident trend. Most derailment causes declined with the greatest reductions in broken rails and welds, track geometry, and other axle and journal defects. Of the few causes that increased, extreme weather was the largest. An updated statistical model of the relationship between track class, traffic density, method of operation, and derailment rate is also developed. Derailments declined uniformly with respect to all combinations of the three factors, indicating a broad general decline across the network. The new model also provides up-to-date derailment rate estimates for use in risk analysis of railroad freight and hazardous materials transportation.