Section 70. The board of aldermen of a city or the selectmen of a town, on petition of fifty legal voters, or on petition executed in accordance with the by-laws or a vote of the directors of a company whose tracks are located in said city or town, after public notice and a hearing as provided in section seven, may grant a location for the extension of the tracks of such company, and prescribe how said tracks shall be laid and the kind of rails, poles, wires and other appliances to be used; but they shall impose no terms or conditions to such grant in addition to those imposed by general laws on such companies in force on October first, eighteen hundred and ninety-eight, or such as may have been imposed in the grant of original location to such company in such city or town subsequent to said date. No such extension of a location shall be valid, until the department, after public notice and a hearing, shall certify that such extension is consistent with the public interest. If the department requires an alteration in such extension before so certifying, it shall notify the board of aldermen or selectmen, granting such extension, of such alteration; and thereafter said board of aldermen or selectmen may amend such extension in accordance with such alteration; provided, that, if such alteration involves a change in the route of the railway, public notice and a hearing shall be given as provided in the case of the original application for an extension; and thereafter the department may, as a part of the original proceedings before it, certify that such extension so amended is consistent with the public interest. An extension, so certified, shall be a valid location, if, within sixty days after the issue of notice of said certification to the company, it files a written acceptance of such extension, executed in accordance with its by-laws or a vote of its directors, with the board of aldermen or selectmen. An extension granted by a board of aldermen or selectmen, but refused certification hereunder by the department, or not accepted as hereinbefore provided, shall be void.