In all cases when any corporation organized under this act to induce aid in its construction, either by donation or subscription to its capital stock, shall desire to fix the rates for any period of time for the transportation of passengers or freight, such corporation may adopt a resolution fixing such rates, and the time for which the same is to be fixed, and have the same recorded in the office of the recorder in the several counties through which the road is proposed to be run; and during the time for which they are fixed, said rates shall in no case be amended by said corporation or its successors: Provided, that said rates shall not exceed the rates allowed by law.