North American railroads are experiencing rapid growth in traffic demand and increasingly need to expand capacity to accommodate it. Efficient planning of new capacity requires understanding how the mixture of traffic interacts to affect capacity. Different train types can have substantially different operating characteristics, including maximum speed, power-to-ton ratio, and dispatching priority. Heterogeneity in the mix of characteristics of different train types creates greater delays than are created if traffic is homogeneous. Train dispatching simulation software was used to analyze the effect of various combinations of intermodal, unit, manifest, and passenger trains on a hypothetical, signalized, single-track line with characteristics typical of a North American railroad subdivision. Analyses included the influence on delay by various traffic and train characteristics. As has been shown by previous investigators, heterogeneity increases delay, but different types of heterogeneity had differing effects, which has implications for capacity planning. This paper attempts to provide a better understanding of the impacts of various aspects of train type heterogeneity to enable more effective planning and efficient rail operations. The results also suggest certain operating strategies that may reduce the delays caused by train type heterogeneity.