Freight train derailment rate has declined substantially over the past decade. Although various aspects of this improvement in train safety have been studied, there has been only limited research examining the effect of train loading condition on derailment occurrence, causes and severity. Unit trains operate loaded in one direction and return empty, and their operation has become more frequent over the past several decades transporting a variety of bulk products. This paper describes research in which an algorithm was developed to identify mainline derailments of loaded and empty unit trains in the US DOT Federal Railroad Administration database. This process was used to develop a dataset of these incidents for the period 2001 to 2015. The number of derailments of loaded and empty trains, the principal causes of these derailments, and their average severity in terms of number of cars derailed are quantified and described.