The officers and employees of the Operations and Safety Section of the Transportation Division of the commission, shall, when in the performance of their official duties, have the right to 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, whether such railroad, car, vessel, or other vehicle is used for the transportation of passengers or freight, and regardless of its class. Such officers and employees when in the performance of their official duties shall have free access to any portion of any train, including the engine or any other units of rolling stock, and to all portions of any vessel or other vehicle of any common carrier used for the transportation of passengers or freight.