Ala. Code § 37-8-3

The agent or employee who locally, in any county, violates any of the provisions of this Code relating to the Public Service Commission or any of the rules or orders of the commission, in pursuance of the direction or authority of his superior officer or agent of the common carrier, may be called as a witness and be compelled to testify, showing the authority by which he acted, and such testimony shall not be used against him, nor shall he thereafter be subject to indictment for said offense.