Section 61. A commission of three disinterested persons, appointed as provided in the following section, shall determine which party shall carry such decision into effect and which party shall pay the charges and expenses of making such alteration and the future charges for keeping such bridge or crossing and the approaches thereto in repair, as well as the costs of the application to the county commissioners, or the department, and of the hearing before said commission; and it may apportion all such charges, expenses and costs between the railroad corporation, the railway company having tracks on said way, and the counties, cities or towns where said crossing is situated and other cities and towns which may be specially benefited; and if the crossing and its approaches are in direct continuation or a part of a state highway, the commonwealth may be included in such apportionment and its share shall be paid from the annual appropriation for maintenance and repair of state highways; provided, that the parties in interest may waive the appointment of the commission and determine the foregoing by written agreement to be filed in the proceeding. If a railway company is authorized to lay and use tracks upon any bridge in a highway built, repaired or altered as above provided for, or the approaches to which are altered or improved as above provided for, the said commission shall determine what part of the charges and expenses of making such changes or improvements, or of keeping such bridge or crossing and approaches in good condition, shall be paid by said railway company.