Abstract: Lightweight composite structural articles having a shell and core comprising a specially-formulated resin, and methods of manufacturing the same. In particular embodiments, the present invention relates to a composite railroad tie comprising an outer fiber-reinforced polymer shell and a unique core formed from a thermoexpandable resin mixture with microcapsules, which railroad tie is manufactured by extruding or pultruding the shell and core through a specially adapted die having a graded cavity, allowing for controlled expansion of the core as the shell and core are passed through the die.