SECTION 19. That the principal office of the Commission shall be in the city of Washington, where its general session shall be held; but whenever the convenience of the public or the parties may be promoted or delay or expenses prevented thereby, the Commission may hold special sessions in any part of the United States. It may, by one or more of the Commissioners, prosecute any inquiry necessary to its duties, in any part of the United States, into any matter or question of fact pertaining to the business of any common carrier subject to the provisions of this Act.