North American freight railroads are expected to experience increasing capacity constraints across their networks. To help plan for this increased traffic, railroads use simulation software to analyze the benefits of capacity expansion projects. Simultaneous operation of heterogeneous traffic further increases delay relative to additional homogenous traffic. Additional passenger trains can cause more delays to freight trains than additional freight trains. Rail Traffic Controller (RTC) was used to run simulations with varying mixes of unit freight and passenger trains operating at various speeds on a double track configuration. Basic assumptions on the relative difference in priority between train types lead to drastically different results on the impact of adding higher priority trains. This assumption dictates whether the track in the opposing direction should be used for overtake maneuvers. Also, higher speed differentials between train types can result in higher delays as faster trains catch up to slower trains more quickly. These analyses will help planners improve their understanding of the tradeoff in capacity due to operation of different types of trains at different priorities and speeds.