Section 15. Except as provided by section two hundred of chapter one hundred and sixty, no common carrier shall, directly or indirectly, issue or give any free service, free tickets, free pass or free transportation for passengers or property between points within the commonwealth; but this section shall not prohibit any railroad corporation or railway company from giving free or reduced rate service to policemen, letter carriers and firemen while in uniform or engaged in the discharge of their duties, or prohibit any common carrier from giving free or reduced rate service to its employees, or in cases of public emergency, or for such charitable purposes as may be approved by the department, nor prohibit any telephone or telegraph company, unless the department shall otherwise order, from giving service at reduced rates to the commonwealth or to any city or town; nor shall this section prohibit the commissioners of the department, its experts, inspectors and counsel from being transported over the railroads and the railways of the commonwealth free of charge while engaged in the performance of their duties; nor shall this section prohibit the giving by any such common carrier of free or reduced rate service to the classes defined and provided for in the act of congress entitled ''An act to regulate commerce'' and acts amendatory thereof, or to a blind person identified by a certification of registration under section one hundred and thirty-five of chapter six from the commission of the blind.