The rail yards located inside Tacoma's Ocean Container Terminals are newer than Seattle's, and include design improvements such as 6 to 10 long tracks each, with air compressors to pre - charge train brake lines, and room for Strads, or even extension Top Picks ( @ WUT ) that can load 2 tracks from the same side. Tacoma Rail does an efficient job of switching the well cars, ( except at TCT, where they like to block the ONLY access road into the facility, during open gate hours ), often using U.P. road power and even adding DMU power too. Same too with BNSF power @ PCT / Evergreen. TTX even has local service & repair crews in these rail yards too.
Also, excepting the TCT deal I mentioned above, the Port of Tacoma road system has been built out to minimize rail blocking of streets, while Seattle Terminal 18 only allows BNSF to switch at night, which includes blocking Lower Spokane Street between Harbor Island & West Seattle.
BOB2 & FUD are spot - on with their mentions of actual import container flow & logistics. It is common for my employer to build loads from items out of 2 or 3 import ocean containers ( mostly 40' X 96" wide X 9'6" high, not always cubed - out ), into 53' X 102" wide Hi - Cube domestic rail cans ( JB Hunt, Schneider, Hub Group, etc ), then have local dray haulers take these cans to the rail yards in Seattle or Tacoma, and it's Happy Trails !!!