North American freight railroads typically exhibit flexible operations with variable train schedules where freight trains are dispatched when filled to length or weight capacity, and train dispatchers resolve train conflicts in real time, making it an ideal approach for low-cost transportation of bulk commodities. Recently, North American railways have experienced a substantial decline in demand for bulk transportation of coal while premium intermodal traffic that must operate on more structured schedules has reached record levels. On the predominantly single-track railway network in North America, operating a mixture of trains with differing schedule flexibility creates fluctuating patterns of train meets that increase train delay, constrain capacity, and decrease utilization of capital-intensive track infrastructure. This study investigates the relationship between line capacity, amount of schedule flexibility, level of service, and the proportion of flexible and scheduled trains operating on a rail line. Using the Rail Traffic Controller software, different combinations of these factors are simulated to determine the capacity of representative North American rail corridors with heavy haul and priority intermodal traffic. The simulation results suggest that introducing more structured schedules for heavy haul traffic is an effective alternative to investment in additional second main track infrastructure to gain sufficient capacity for growing scheduled intermodal traffic on traditional bulk commodity freight corridors.