The (fairly) new D street bridge in Tacoma is a good place. It has wide sidewalks on both sides and the track goes east/west, so you can work the sun angles all day. This is just north of the west end of Freighthouse square.

Steilacoom is very good between town and Sunnyside Park just north of town. This is especially good in the morning, although later in the day you can shoot from the ferry dock right in town. There are other spots in Steilacoom, too, that can be sniffed out.

The Puyallup Sounder station is good because the tracks here also run east/west. Be on the south side. I haven't been there for awhile, but I would certainly check in with the security guard when arriving.

In Seattle there are spots along Myrtle Edwards Park, just north of the downtown waterfront.

And north of Seattle try MP 18 near the Edmonds ferry dock. I suspect that is best in the morning.

Good luck.