Truck, rail, water, air, and pipeline modes each serve a distinct share of the freight transportation market. The current allocation of freight by mode is the product of technologic, economic, and regulatory frameworks, and a variety of factors -- price, speed, reliability, accessibility, visibility, security, and safety -- influence mode. Based on a comprehensive literature review, this reportconsiders how analytical methods can be used to project future modal shares and offers insights on federal policy decisions with the potential to prompt shifts to energy-efficient, low-emission modes. There are substantial opportunities to reduce the energy used for freight transportation, but it will be difficult to shift large volumes from one mode to another without imposing considerableadditional costs on businesses and consumers. This report explores federal government actions that could help trigger the shifts in modal shares needed to reduce energy consumption and emissions. This is one in a series of reports produced as a result of the Transportation Energy Futures project, a Department of Energy-sponsored multi-agency effort to pinpoint underexplored strategies forreducing GHGs and petroleum dependence related to transportation.