To minimize costs, railways carefully match route infrastructure investment to projected traffic levels. Under structured operations according to a rigid timetable, the optimal amount of infrastructure for a given traffic volume can be determined with various models. However, North American railways use flexible operations for which planned train departure times can vary. Schedule flexibility increases the number of possible meet locations on a single track, which can lead to a combination of increased delays and infrastructure investment. When industry practitioners plan rail line capacity for flexible operations, they rarely optimize schedule flexibility and infrastructure investment simultaneously for a given level of service. To increase knowledge of the relationship between these factors, the research reported in this paper simulated operations on two representative single-track routes with rail traffic controller software. A baseline minimum-delay schedule was developed for each route under an initial infrastructure. The experiment design introduced schedule flexibility and infrastructure expansion to examine the interaction between these factors and train delay response. The results suggested that, for a given infrastructure configuration, an immediate increase in train delay occurred with small amounts of schedule flexibility. After this increase, routes became insensitive to further increases in schedule flexibility. Similarly, to maintain a level of service required substantial infrastructure investment for small amounts of schedule flexibility.