The public utilities commissioners and the attorney, secretary, other officers, and employees of the public utilities commission may, when in the performance of their official duties, pass free of charge on all railroads, cars, vessels, and other vehicles of every common carrier, subject to control or regulation by the commission, between points within this state, and such persons shall not be denied the right to travel upon any railroad, car, vessel, or other vehicle of such common carrier, whether such railroad, car, vessel, or other vehicle is used for the transportation of passengers or freight, and regardless of its class.