No extension or enlargement of any franchise, or grant, or rights, or powers, previously granted to any corporation, person or association of persons, shall be made except in the manner, and subject to all the conditions herein provided in this chapter for the making of original grants and franchises; provided, however, that the provisions of this chapter shall not apply to the granting by ordinance of revocable licenses or privileges for side track or switch privileges, to railway companies, for the purpose of reaching and affording railway connection and switch privileges to the owners or users of any industrial plant, warehouse or mercantile establishment, it being the intention to permit the city to grant such revocable license or privilege to railway companies whenever, in its judgment, the same is expedient, necessary or advisable, and application for such a privilege is accompanied by the assent, in writing, of the owners of the major part of the frontage of the lots or tracts of land of the blocks fronting on each side of any street, or parts of a street, over or on which it is desired to lay or construct such side tracks or switches.