When the board of county commissioners is of the opinion that the welfare and convenience of the public will be served by the use of a bridge designed to carry the tracks of a railroad company, a union depot company, or any of said companies, and general highway traffic, the board may contract with such companies for the joint construction, use, and maintenance of such a bridge with provision for general highway traffic, in conjunction with and as a part of such railroad bridge, either at, above, or below the grade or elevation at which said railroad tracks are constructed.