Wash. Rev. Code § 39.34.085
In addition to the other powers granted by chapter 39.34 RCW, one or more cities or towns or a county, or any combination thereof, may enter into agreements with each other or with a public transportation agency of a contiguous state, or contiguous Canadian province, to allow a city or such other transportation agency to operate bus service for the transportation of the general public within the territorial boundaries of such city and/or county or to allow such city and/or county to operate such bus service within the jurisdiction of such other public agency when no such existing bus certificate of public convenience and necessity has been authorized by the Washington utilities and transportation commission: PROVIDED, HOWEVER, That such transportation may extend beyond the territorial boundaries of either party to the agreement if the agreement so provides, and if such service is not in conflict with existing bus service authorized by the Washington utilities and transportation commission. The provisions of this section shall be cumulative and nonexclusive and shall not affect any other right granted by this chapter or any other provision of law.
RCW 39.34.085