The Commissioner of Transportation may, in the absence of any application therefor, when in the commissioner’s opinion public safety requires an alteration in any highway crossed at grade by a railroad or by railroads belonging to or operated by more than one company, after a hearing had upon such notice as the commissioner deems reasonable to the company or companies owning or operating such railroad or railroads and to the selectmen of the town, mayor of the city or warden of the borough within which such highway is situated and to the owners of the land adjoining such crossing, order such alterations in such highway as the commissioner deems best, and shall determine and direct by whom such alterations shall be made, at whose expense and within what time; provided, in all cases arising under this section, one-fourth of the expense, including damages and special damages as aforesaid, shall be paid by the state and the remainder shall be assessed upon the railroad company or companies benefited by such order; and provided such alterations as are thus made at the primary instance of the commissioner shall not be ordered so as to direct the construction of more than one bridge in any one year on any one railroad. Railroad companies may take land for the purpose of this section. No land shall be taken by any railroad company for the purpose mentioned in this section, except such as the commissioner finds to be necessary for such purpose; but no such taking need be based upon any special finding that public necessity and convenience require such taking.