Being that I work halfway between SEA and PDX I do this trip often by planes, trains, and automobile. The drive from PDX to Centralia is delightful, SEA to Centralia not so much, however road is in good shape both ways. The Amtrak from PDX is easy and inexpensive if you have the time to take the light rail (one connection) to a block away from Union Station in Portland. The flying part is the best between PDX and SEA, my nose is glued to the window, even on cloudy days because seeing the tops of the mountains is still exciting after all these years.