The motivation for providing shuttle and unit train service1 lies in a simple realization – that freight rail transportation is most cost efficient when a large number of railcars move together on a single train. As the number of railcars on a train increases, the costs of rail transportation are spread over more railcars, resulting in a lower cost per railcar; hence, railroads aim to move as many railcars as possible on a single train. In order to obtain the cost efficiencies associated with large trains, railroads have historically consolidated railcars at classification yards. Under conventional rail service, single or multiple railcars are typically moved from their origin to a nearby classification yard where they are grouped with other railcars traveling to a similar destination. Railcars often travel through multiple classification yards before reaching their destination. The disassembly and reassembly of railcars at classification yards is a time and resource consuming process, subject to considerable variability, and hence is a source of unreliability in service times. Historically, railroads and shippers have accepted the inefficiencies of classification yards as the price of railcar consolidation (Keaton, 1991). However, the introduction of shuttle train service for bulk commodities, such as coal and grain, allowed railroads to move a large number of railcars directly from origin to destination, thus bypassing classification yards.