A common carrier shall be subject to the jurisdiction of the Transportation Board and, while so operating, to all reasonable rules as the Board prescribes with respect to routes, fares, schedules, continuity of service, and the convenience and safety of passengers and the public. The operation of a motor vehicle the principal purpose of which is the carrying of mail under contract with the U.S. government shall not be construed as being engaged in the business of operating a motor bus within the meaning of this section and section 1820 of this title, notwithstanding incidental transportation of passengers for hire. (Added 1985, No. 224 (Adj. Sess.), § 1.)