Siding Spacing and the Incremental Capacity of the Transition From Single to Double Track

Abstract

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 build-out 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.

Operational Schedule Flexibility and Infrastructure Investment: Capacity Trade-Off on Single-Track Railways

“…For Model 1, the initial sparse single-track infrastructure at 40-mi spacing (corresponded to factor level zero) was incrementally expanded by adding between one and 18 new passing sidings along the route. Passing sidings were installed midway between existing sidings and were added along the route in a symmetric and uniform manner according to the methodology of other research into the incremental capacity of single track (16,17). Symmetric placement of sidings relative to the center of the route was maintained.…”

Capacity of Single-Track Railway Lines with Short Sidings to Support Operation of Long Freight Trains

“…Future work will investigate a broader range of total freight throughput values and ratios of long to short train lengths to determine whether the free-flow point of 50% long sidings varies or is a fundamental property of single-track lines. The investigated routes are idealized and, as with other delay and capacity relationships, the trends between percent railcars in long trains and percent long sidings may not hold for routes with uneven siding spacing (11). This research also considered only routes with homogeneous freight traffic.…”