A lightweight, reinforced composite structural element is disclosed, which includes a hollow housing, interior reinforcement, a filling of polyurethane foam, and a substantially uniform outer coating of a high-solid protective coating. The reinforced composite structural element can be used as a railroad tie and for other construction uses. The structural element is formed with a housing having end caps or ends sealed by flaps that are connected with and bendable from the housing material to create a sealed container with openings for injection of the foam into cavities of the housing formed by the interior reinforcement.