Wash. Rev. Code § 81.28.200
A common carrier, subject to regulation by the commission as to rates and service and this title, shall not charge or receive any greater compensation in the aggregate for the transportation of persons or a like kind of property for a shorter distance than for a longer distance over the same line in the same direction, the shorter distance being included within the longer distance, or to charge any greater compensation as a through rate than the aggregate of the intermediate rates subject to this title. The common carriers may not charge and receive as great a compensation for a shorter as for a longer distance or haul. Upon the application of a common carrier, the commission may by order authorize the common carrier to charge less for a longer distance than for a shorter distance for the transportation of persons or property in special cases after investigation by the commission, but the order must specify and prescribe the extent to which the common carrier making the application is relieved from the operation of this section. Only to the extent so specified and prescribed is any common carrier relieved from the operation and requirements of this section.
RCW 81.28.200