§ 15.   Railways and rail companies declared public highways and common carriers--Regulation of rates. Railways heretofore constructed or that may hereafter be constructed, in this state are hereby declared public highways, and all railroad and transportation companies are declared to be common carriers and subject to legislative control; and the Legislature shall have power to enact laws regulating and controlling the rates of charges for the transportation of passengers and freight as such common carriers from one point to another in this state.