(a) The Public Service Commission, upon its own motion or upon the written application of any transportation company, and under any rules which it may prescribe, may permit, from time to time, such transportation company to establish any special rate or rates for the transportation of specific commodities in carloads from specified points or within specified zones or distances to the plants or points of destination where are located the plants of any person, firm or corporation who is now, or may hereafter be, engaged in any industrial or agricultural enterprise in this state, for the purpose of encouraging the establishment or aiding in the development or continued and successful operation of such industrial or agricultural enterprise; and any special rate or rates so established for any particular industrial or agricultural enterprise shall apply and be given all persons, firms and corporations engaged in the same enterprise within said zone or distance.

(b) Said special rates shall first be approved by the commission and be published as it may direct, and a full, true and correct statement in writing of such special rate or rates shall, in each instance, be first filed with said commission, together with a sworn copy of any special contract made with the shipper with respect to such special rates, before the same shall go into effect; and the said commission may at any time revoke such permission and any special rate or rates established thereunder. Any rate or rates so established after compliance with the provisions hereinabove prescribed shall be the lawful rates for the service performed and shall not be construed to be an unjust or unlawful discrimination.