RCW 81.28.180

A common carrier subject to regulation by the commission as to rates and service shall not, directly or indirectly, by any special rate, rebate, drawback, or other device or method, charge, demand, collect, or receive from any person or corporation a greater or lesser compensation for any service rendered or to be rendered in the transportation of persons or property, except as authorized in this title, than it charges, demands, collects, or receives from any person or corporation for doing a like and contemporaneous service in the transportation of a like kind of traffic under the same or substantially similar circumstances and conditions.