Section 73. If a bridge upon which a company is authorized to lay and use tracks is being or is to be altered, rebuilt, improved or repaired, the board of aldermen of a city or the selectmen of a town on petition of the president or a majority of the directors of such company may grant a temporary location for the extension of the tracks of such company in any public way in such city or town so as to provide such connection between the existing tracks of such company upon either side of such bridge as will prevent any interruption of proper transportation facilities by reason of such alteration, improvement, rebuilding or repair, and may prescribe how said tracks shall be laid and the kind of rails, poles, wires and other appliances to be used. Such location shall not be valid unless the board of aldermen or selectmen file with the clerk of the city or town concerned a certificate that the granting of the location is consistent with the public interest. In no case shall a location be valid unless, within thirty days after the filing of the said certificate or after the entering of a decree by the department, a majority of the directors of the company shall file with the body granting the location a written acceptance thereof.