If you have a car, then there are lots of options in and around Seattle. For UP stack train action, you can park at Home Depot on 4th street south, next to the building with the Starbucks HQ. if you get there early enough you'll be trackside, as their parkIng lot abutted the yard. Seattle' s union station on King Street recently under went some major renovations, and there are several overpasses over the tracks for watching Amtrsk and Sounder trains.
Moving north, the BNSF Interbay Yard is pretty accessible from a variety of areas - the shops complex is right along one of the Magnolia neighborhood 's main drags. You can also see trains from the grounds of the Ballard Locks, and Golden Gardens Park. I'll try to post some. Bing maps tomorrow when I'm not On my phone.