Section 18. Subject to the powers of the department to regulate and prescribe rates and charges, a common carrier may make commodity, transit or other classes of rates. The furnishing by any common carrier of any service at the rates and upon the terms and conditions provided for in any existing contract executed prior to July first, nineteen hundred and thirteen, shall not constitute a discrimination unless the department so determines. The department shall not be prevented from taking such action as it deems proper by any commitment or agreement of a common carrier entered into by reason of any requirement or recommendation of any board or public officers acting under delegated authority from the general court prior to July first, nineteen hundred and thirteen. Unless the department determines otherwise, common carriers shall be permitted, whether required to do so by law or not, to issue mileage, workingmen's, excursion, school or commutation passenger tickets, or reduced rate tickets for the transportation of children under twelve years of age or of pupils attending school, or joint interchangeable mileage tickets, with special privileges as to the amount of free baggage which may be carried under mileage tickets of five hundred miles or more. All season tickets, before issuance, shall be subject to the approval of the department as to the form thereof and the conditions named therein.