Mich. Comp. Laws § 123.335

Sec. 5. The several public corporations in which said system lies may, at any time before or after the date specified, enter into a contract providing, in lieu of the provisions of section 4 of this act, for the division of such system, between the several public corporations, or for its operation as a single unit either by 1 of the public corporations as agent for all, or by a joint board of commissioners, 1 or more of whom shall be appointed by the legislative or governing body of each public corporation, and 1 or more shall be elected by such commissioners or by some disinterested agency.