Wash. Rev. Code § 81.04.460
It shall be the duty of the commission to enforce the provisions of this title and all other acts of this state affecting public service companies, the enforcement of which is not specifically vested in some other officer or tribunal. Any employee of the commission may, without a warrant, arrest any person found violating in his or her presence any provision of this title, or any rule or regulation adopted by the commission: PROVIDED, That each such employee shall be first specifically designated in writing by the commission or a member thereof as having been found to be a fit and proper person to exercise such authority. Upon being so designated, such person shall be a peace officer and a police officer for the purposes herein mentioned.
RCW 81.04.460