When the Commissioner of Transportation, in accepting the layout of any railroad company, has in such acceptance provided that portions of such railroad shall not be constructed until certain highways have been relocated or changed by such company, and the obligation of repairing or maintaining the whole or any part of such highways is imposed upon any person or corporation other than the town, city or borough within which such highway may be located, such provision shall be binding upon the company and it shall maintain and repair such highway in the same manner and to the same extent that such other person or corporation was bound to repair and maintain the same before such relocation or change. Any such company may use the material and abutments of any existing bridge in the old highway in the construction of a bridge in the substituted highway and shall provide suitable temporary accommodations for public travel over the old highway until the new highway is completed and shall be solely responsible for injuries resulting from its negligence in the matter of such temporary accommodations. The selectmen of any such town may discontinue such parts of the old highway as in their judgment are not of public convenience and necessity.