No member of a train crew, yard crew, or engine crew of a railroad common carrier shall be held personally responsible under, or found guilty of violating, any state laws or municipal ordinances regulating or intended to regulate the occupying or blocking of any street, road, or highway grade crossing by engines or passenger or freight cars, upon reasonable proof by the crew member that the occupying or blocking of the grade crossing was necessary to comply with the orders or instructions, either written or oral, of his employer or of the officers or supervisory officials of the company owning the railroad over which the engine or cars are operated; provided, however, that this Code section shall not relieve the employer or railroad company from any responsibility placed upon such employee or railroad company by any such state laws or municipal ordinances.