A. When the commission finds that additions or improvements to or changes in the existing plant or physical properties of a public service corporation ought reasonably to be made, or that a new structure or structures should be erected, to promote the security or convenience of its employees or the public, the commission shall make and serve an order directing that such changes be made or such structure be erected in the manner and within the time specified in the order. If the commission orders erection of a new structure, it may also fix the site thereof.

B. If any such changes require joint action by two or more public service corporations, the commission shall notify the corporations thereof and that the changes shall be made at their joint cost, whereupon the corporations shall have such reasonable time as the commission grants within which to agree upon the division of cost. If at the expiration of the time the corporations fail to file with the commission a statement that an agreement is made for a division or apportionment of the cost or expense of the change, the commission may make an order fixing the proportion of the cost or expense to be borne by each corporation, and the manner of payment.