If the elevating machines or hoisting apparatus are used within a building in the course of constructing for the purpose of lifting material to be used in such construction, the contractor or owner shall cause the shafts or openings on floor where material is loaded to be completely enclosed on all sides; except opening not over eight feet high and the width of the elevating machines for loading purposes. On the other floors the shafts and all other openings shall be enclosed or fenced in on all sides by a substantial barrier or railing at least three feet in height; provided, however, that nothing in sections 292.450 to 292.540 shall apply to railroad corporations or companies who are using their own employees in the construction or repairing of any structure mentioned herein.