Rail transport can be more efficient than other modes of transportation when measured in ton-miles or tonne-kilometers hauled per unit of energy consumed. Bulk goods (for example, coal) often yield the greatest savings, especially when transported across vast distances. However, rail transport is less flexible[vague] than highway transport, thus much freight is moved by vehicle, even over vast distances. When moving goods by rail, transshipment fees are frequently incurred, especially when the shipper or receiver lacks direct rail access. These expenses may exceed those of operating the train, a factor that practises like containerization, trailer-on-flatcar, and rolling highway seek to reduce.