((a)) ** .—** A carrier providing transportation subject to the jurisdiction of the Board under subtitle IV shall designate an agent on whom service of process in an action before a district court may be made. Except as otherwise provided, process in an action before a district court shall be served on the designated agent of that carrier at the office or usual place of residence of that agent. If the carrier does not have a designated agent, service may be made by posting the notice in the office of the Board.

((b)) ** .—** A designation under this section may be changed at any time in the same manner as originally made.