The North American freight railroad network is projected to experience rising transportation demand in the coming decades, leading to increased congestion along many rail corridors. Increased interest in expanded passenger service on shared rail corridors will also create additional capacity demand. However, the nation's rail lines are still predominantly single track with passing sidings, making double track installation a vital capacity upgrade measure to sustain future volumes. Since increasing capacity through double track installation requires significant capital investment, the second main track must be allocated along a line in an optimal manner to provide maximum return on investment. An approach of investing in the least costly segments first may yield good results, but only if the benefits for each segment are equal. This research seeks to identify if the benefit of double track varies between bottleneck segments, and if there are compounding benefits of double track between adjacent passing sidings. Previous research has explored the allocation of double track on an idealized line with evenly spaced passing sidings. Due to numerous physical and engineering constraints, existing lines often exhibit a mixture of siding spacing with single-track bottleneck sections of varying length. To investigate the incremental capacity of adding double-track segments to a route with variable siding spacing, several buildout strategies are tested on a representative subdivision under random, mixed freight and passenger traffic via Rail Traffic Controller simulation software. The presented results highlight the most effective method, based on train delay, of incremental single to double track expansion and the potential differences in benefit between strategies. The linear delay reduction characteristics of single-to-double track mainlines vary based on the initial spatial arrangement of passing sidings and amount of second main track installed. These results further the understanding of relationships between infrastructure location and freight delay, thereby serving as a guideline for the sustainable expansion of existing rail corridors in anticipation of future demands. While railroads must consider many factors in selecting capital expansion projects, these guidelines can streamline the decision process by helping to quickly identify the projects with the most potential for more detailed engineering evaluation. The methodology presented can eventually be incorporated into analyzing the progressions from double to triple track lines.