A slight correction to your post, Tom. The Union Pacific only used Milwaukee Road tracks from Tacoma ( Reservation ) to Black River Jct. in Tukwila. From Black River North to Argo U.P. had their own tracks, while Milwaukee used Pacific Coast RR trackage rights. These trackage rights were also used from Black River through Renton out to Maple Valley.
BTW, the current issue ( 3rd Qtr 2009 ) of The Milwaukee Railroader magazine has several excellent overhead shots of Milwaukee's Seattle yards & facilities circa 1967, as well as maps explaining area RR's & ownership.