Wash. Rev. Code § 35.58.180
In addition to the powers specifically granted by this chapter a metropolitan municipal corporation shall have all powers which are necessary to carry out the purposes of the metropolitan municipal corporation and to perform authorized metropolitan functions. A metropolitan municipal corporation may contract with the United States or any agency thereof, any state or agency thereof, any other metropolitan municipal corporation, any county, city, special district, or governmental agency and any private person, firm or corporation for the purpose of receiving gifts or grants or securing loans or advances for preliminary planning and feasibility studies, or for the design, construction or operation of metropolitan facilities and a metropolitan municipal corporation may contract with any governmental agency or with any private person, firm or corporation for the use by either contracting party of all or any part of the facilities, structures, lands, interests in lands, air rights over lands and rights-of-way of all kinds which are owned, leased or held by the other party and for the purpose of planning, constructing or operating any facility or performing any service which the metropolitan municipal corporation may be authorized to operate or perform, on such terms as may be agreed upon by the contracting parties: PROVIDED, That before any contract for the lease or operation of any metropolitan public transportation facilities shall be let to any private person, firm or corporation, a general schedule of rental rates for bus equipment with or without drivers shall be publicly posted applicable to all private certificated carriers, and for other facilities competitive bids shall first be called upon such notice, bidder qualifications and bid conditions as the metropolitan council shall determine.
A metropolitan municipal corporation may sue and be sued in its corporate capacity in all courts and in all proceedings.
RCW 35.58.180