As of 2011, Seattle does not have an underground subway system.
Seattle does canonically have the Ork Underground. Seattle does have roads and trains going through tunnels. See I 90 between Downtown and Bellevue. I believe there is a rail tunnel on the west side of Tacoma that dumps over into the docks area. I think there is another tunnel going under 405 in Bellevue, adjacent to the Cavilard research center. I-5 goes under the Convention Center downtown. The Arcology, and Aztechnology pyramid both have deep basements and parking lots.
If you're looking for a 'Subway Platform' set piece, they are available.