Between Aug 8-12, I'm going to be in SEA. Where are the best places to photograph passenger operations? How is Tukwila? I'm going to be staying in Sea-Tac.
The station at Tukwila is good, if not particularly scenic. One advantage is that the BNSF and UP are parallel there and both can be photographed from the southbound platform. You'll see plenty of Sounders, Cascades, and the Coast Starlight.
I'd recommend taking the Link light rail into the city from SeaTac. It drops you right in Union Station, former home of UP and MILW long distance trains and a wonderful building. A short walk across the street is King Street Station, Amtrak's current station and the former home of NP and GN. The interior is magnificent. Slightly south of the station is Holgate Street, which bisects the coach yard and has wide sidewalks for rail viewing. It isn't in the best of areas, though, so be on guard.
North of King Street Station and the GN tunnel is the waterfront, where the tracks run very close to the water. The Olympic Sculpture Park has a footbridge over the tracks that is excellent, particularly in the evening. You'll see Sounder, Amtrak Cascades, and the Empire Builder here. North of the waterfront is Interbay/Balmer yard, which has overpasses and nearby streets that offer views of the roundhouse and engine service tracks. A trail allows for view of the small hump. Another excellent location is the Salmon Bay Bridge as viewed from the Chittenden Locks.