The railway industry throughout the world continues to emphasize the importance of developing innovative trackbed design technologies for both heavy tonnage freight lines and high-speed passenger lines. The purposes are to achieve high levels of track geometric standards for safe and efficient train operations while minimizing long-term track maintenance costs and extending track component service lives. During the past several decades designs incorporating a layer of asphalt (or bituminous) paving material, similar to a highway pavement asphalt base layer, as a portion of the railway track support structure have steadily increased until it is becoming a common or standard practice.

This technology has demonstrated applications for the construction of numerous new high-speed passenger lines in Europe and Asia. Asphalt trackbeds have been primarily limited to heavy tonnage freight lines in the United States, most often for maintenance/rehabilitation of special trackworks or capacity improvements of existing lines. Increasingly, asphalt is also specified for new urban tail transit/commuter lines.

The primary documented benefits described in this three-part paper are to: 1) provide additional support to improve load distributing capabilities of the trackbed layered components, 2) decrease load-induced subgrade pressures, 3) increase confinement for the ballast, 4) improve and control drainage, 5) maintain consistently low moisture contents in the subgrade, 6) insure maintenance of specified track geometric properties for heavy tonnage freight lines and high-speed passenger lines, and 7) decrease subsequent expenditures for trackbed maintenance and component replacement costs.

In Part I, various factors are discussed that are considerations in the design phases. Illustrations of the trackbed/roadbed components, construction phases, and finished projects are presented for various asphalt trackbed applications in several countries, including Italy, Austria, and France. Part II describes applications in the United States. Part III presents performance-based tests and analyses.