The action or practice of riding or carrying on piggyback; the action or practice of mounting or carrying one thing on another thing; (figurative)…

trailer on flatcar (with reference to a type of freight container).

trailer-on-flatcar, used to denote a system of freight transport whereby trailers (and other unaccompanied road freight vehicles) are carried on…

figurative. A system of containerized freight transportation by railway in which a loaded road trailer complete with wheels is carried on a flat…