Wash. Rev. Code § 35.58.260
If a metropolitan municipal corporation shall be authorized to perform the metropolitan transportation function, it shall, upon the effective date of the assumption of such power, have and exercise all rights with respect to the construction, acquisition, maintenance, operation, extension, alteration, repair, control and management of passenger transportation which any component city shall have been previously empowered to exercise and, except as provided in RCW 35.21.925 and 36.73.180, such powers shall not thereafter be exercised by such component cities without the consent of the metropolitan municipal corporation: PROVIDED, That any city owning and operating a public transportation system on such effective date may continue to operate such system within such city until such system shall have been acquired by the metropolitan municipal corporation and a metropolitan municipal corporation may not acquire such system without the consent of the city council of such city.
RCW 35.58.260