Section 3. If a person desiring to operate any motor vehicle for the purposes and in the manner aforesaid over a route covering at least twenty miles holds a license therefor in the terminal municipalities and also a license in all but one of the intervening municipalities, or, in case seven or more municipalities intervene, in all but one or two thereof, the department of public utilities, hereinafter in this chapter called the department, on petition of such person, shall act as the licensing authority in the one or two municipalities, as the case may be, in which such person’s application for such an original license has not been favorably acted upon within three months after the filing thereof. The department, before issuing such a license, shall give a public hearing thereon after notice to the licensing authority of such a municipality, and if the department finds that public convenience and necessity require that the applicant be allowed to operate motor vehicles through such a municipality and over a route as aforesaid, it may issue a license therefor and shall specify therein the route or routes over which such motor vehicles shall be operated therein, but operation under such a license shall be limited to through traffic without stopping in such municipality for taking on or discharging passengers, except in case of a railroad or railway company operating a bus line as a part of its system.