A common carrier of persons must provide every passenger with a seat. He must not overload his vehicle by receiving and carrying more passengers than its rated capacity allows. This section shall not apply, however, to any city, county, city and county that operates a transportation system, or to any passenger stage corporation or street railroad corporation, as defined in Sections 226 and 232, respectively, of the Public Utilities Code, which is subject to the jursidiction of the Public Utilities Commission.