Some answers for MATT, with apologies to TaxMan. (TaxMax ??? )
-- As noted above, the current arrangement for Link Light Rail @ Sea Tac Airport is quite inconvenient, especially with family and / or luggage in tow. Suggest a STITA cab; they have a flat rate ( $20. I think ) from the airport to any downtown destination.
-- You didn't mention where you are staying downtown, however the bus system & Link transit tunnel make going North / South easy. Traveling East / West is a different story; from the waterfront East to Interstate 5 is UPhill. Remember 2nd Ave is one way South, 4th Ave is one way North.
-- Several good train watching spots are King St. Station & South Portal of the Railroad tunnel under downtown. Photo angles are tight unless you go South to Royal Brougham Ave X - ing; a bonus here is a view of North End of Amtrak coach yard from Edgar Martinez Blvd overpass @ South side of Safeco Field. ( Mariners should be out of playoffs by then, so crowds will be down. )
-- Further South, 4th Ave. South bridge over Ewe - Pee Argo Yard, the " Coach Wye " visible from West sidewalk. This location on BNSF Main is where a wye serves as access to BNSF Stacy St. Intermodal yard; as the name implies Amtrak turns Trains 7 / 8 & 11 / 14 here. 1st Ave South also spans the UP yard, Engine facility visible from West sidewalk. To the East, Airport Way spans the South end of UP Yard, also the location of ARGO " Crossing ". Once upon a time BN & UP crossed here, lots of trackwork for Sounder Commuter Trains included buying UP line from Argo South to Black River (Tukwila ), new crossovers & signals visible from East sidewalk on Airport Way bridge. WARNING: Airport Way has heavy truck traffic ( including me! ) & narrow sidewalks.
-- Alaska RR barge slips are located on Harbor Island ( UP ), and West Seattle ( BNSF ), both Southwest of downtown. Both have public viewing available; however load / unload operations are tidal dependent & often @ night.
-- Hope this helps...