Amtrak's Seattle facility is the only maintenance base for Sounder Trains; Tacoma performs only light coach cleaning during the nightly layover. As well, Seattle is the major maintenance base for Amtrak in the PNW; Portland performs only light maintenance & car cleaning for the Portland section of the Empire Builder, as well as restocking the lounge car.
The Amtrak Seattle maintenance facility is undergoing an overdue upgrading & enlargement that began with the moving of BNSF's mainlines from the West to the East side of the property; thus allowing Amtrak to switch King St. Station w/o having to cross over the mains. Planned construction will include a car service building, bringing such activities inside for the 1st time in the 16 years I've lived here. You're right, the current facility is cramped; several Sounder trainsets lay over on the South end of the old BNSF mainlines during the day.
Adam's picture was taken from the sidewalk of Holgate St., which bisects the Amtrak facility. There is still an ongoing conversation between Amtrak & City of Seattle over efforts to close Holgate St crossing. Nice blues in your photo, Adam. I'd say you were using a Polarizer on your lens, but that's probably old school.