Any company may so alter the location of its road as to add to the number of its main tracks and, for that purpose, with the approval of the Commissioner of Transportation, may take additional land; but, when an additional bridge over a navigable stream is required by an addition to the main tracks, the same shall be constructed in such manner, of such materials and with draws of such width as said commissioner may authorize and direct, and such additional bridge shall be subject to the provisions of section 13b-285.