Use of distributed-power locomotives in heavy-haul service has allowed for greater efficiencies through operation of longer freight trains. In North America, where the majority of main-line routes are single track, the potential economic and operational advantages offered by long trains are constrained by the inadequate length of many existing passing sidings. To implement longer trains, railroad operators may run long trains in a single direction or fleet trains or extend sidings enough to facilitate bidirectional operation of long trains. This research seeks to characterize the interaction between passing siding and train lengths and the subsequent effect on train delay. The operation of a single-track line is simulated under different combinations of freight throughput, percent of long sidings, percent of railcars in long trains, and directional distribution of long trains. The experiment design scenarios are simulated by using Rail Traffic Controller software. Results indicate that routes with roughly 50% of long sidings eliminate any delay-based consequences of running long trains on the route. Thus, to operate with a high percentage of long trains, only half of the sidings on a route need to be extended to maintain the baseline level of service. These results are expanded through discussion of their practical application in planning programs to either extend existing sidings or construct new longer length sidings. The findings can streamline the decision-making process associated with capital expansion projects and further the understanding of relationships between siding and train lengths, thereby facilitating the sustainable expansion of existing rail corridors in anticipation of future demand.