Nothing in this Part or in any byelaws to be made under any enactment extended by this Part shall apply to a building (other than a dwelling-house) belonging to a railway company, or to any company or other public body authorised to construct, maintain, or improve a harbour, pier or dock, or to the owners of any canal or inland navigation, and used by the company, public body, or owners as a part of or in connection with their railway, harbour, pier, dock, canal or inland navigation.