(a) In any proceeding for the enforcement of the provisions of this chapter, whether such proceeding be instituted before the Commission or be begun originally in any district court of the United States, it shall be lawful to include as parties, in addition to the carrier, all persons interested in or affected by the charge, regulation, or practice under consideration, and inquiries, investigations, orders, and decrees may be made with reference to and against such additional parties in the same manner, to the same extent, and subject to the same provisions as are or shall be authorized by law with respect to carriers.

(b) In any suit for the enforcement of an order for the payment of money all parties in whose favor the Commission may have made an award for damages by a single order may be joined as plaintiffs, and all of the carriers parties to such order awarding such damages may be joined as defendants, and such suit may be maintained by such joint plaintiffs and against such joint defendants in any district where any one of such joint plaintiffs could maintain such suit against any one of such joint defendants; and service of process against any one of such defendants as may not be found in the district where the suit is brought may be made in any district where such defendant carrier has its principal operating office. In case of such joint suit, the recovery, if any, may be by judgment in favor of any one of such plaintiffs, against the defendant found to be liable to such plaintiff.