Wash. Rev. Code § 47.08.070
When it appears to the department that any state highway will be benefited or improved by the construction of any public works project, including any urban public transportation system, within the state of Washington by any of the departments of the state of Washington, by the federal government, or by any agency, instrumentality, or municipal corporation of either the state of Washington or the United States, the department is authorized to enter into cooperative agreements with any such state department, with the United States, or with any agency, instrumentality, or municipal corporation of either the state of Washington or the United States, wherein the state of Washington, acting through the department, will participate in the cost of the public works project in such amount as may be determined by the department to be the value of the benefits or improvements to the particular state highway derived from the construction of the public works project. Under any such agreement the department may contribute to the cost of the public works project by making direct payment to the particular state department, federal government, or to any agency, instrumentality, or municipal corporation of either the state or the United States, or any combination thereof, which may be involved in the project, from any funds appropriated to the department and available for highway purposes, or by doing a portion of the project either by day labor or by contract, or in any other manner as may be deemed advisable and necessary by the department.
RCW 47.08.070
Urban public transportation system defined: RCW 47.04.082.