The district may acquire, or contract to acquire, and may construct under contract, or by its own employees, maintain, improve, and operate bridges, abutments, rights-of-way, roads, tunnels, railroads, streetcar lines, interurban lines, telephone and telegraph lines, footpaths, viaducts, tollgates, tollhouses, subways, and all other forms of property necessary or proper for the successful prosecution of the project for which the district was organized, or which may be authorized by the board of directors of the district under its powers, including all machinery or other property useful or necessary to construct, maintain, operate, or otherwise make use of toll bridges and highways, and complete, add to, repair, or otherwise improve, any of the property acquired by it.