Wash. Rev. Code § 35.58.250
Except in accordance with an agreement made as provided herein, upon the effective date on which the metropolitan municipal corporation commences to perform the metropolitan transportation function, no person or private corporation shall operate a local public passenger transportation service within the metropolitan area with the exception of taxis, buses owned or operated by a school district or private school, and buses owned or operated by any corporation or organization solely for the purposes of the corporation or organization and for the use of which no fee or fare is charged.
An agreement may be entered into between the metropolitan municipal corporation and any person or corporation legally operating a local public passenger transportation service wholly within or partly within and partly without the metropolitan area and on said effective date under which such person or corporation may continue to operate such service or any part thereof for such time and upon such terms and conditions as provided in such agreement. Where any such local public passenger transportation service will be required to cease to operate within the metropolitan area, the commission may agree with the owner of such service to purchase the assets used in providing such service, or if no agreement can be reached, the commission shall condemn such assets in the manner provided herein for the condemnation of other properties.
Wherever a privately owned public carrier operates wholly or partly within a metropolitan municipal corporation, the Washington utilities and transportation commission shall continue to exercise jurisdiction over such operation as provided by law.
RCW 35.58.250