The right of way in the photos, was the former BN Prairie line. Which ran from a connection just south of Union Station, to Nisqually, an alternative route to the Pt. Defiance routing. The portion between M St and Union Station has been out of service for many years (track removed in places), as the University of Washington built a campus on both sides of the tracks in downtown Tacoma. The University convinced BNSF that the slow moving infrequent trains (local freights) were a threat to the students. Much of the railroad infrastructure still exists through the campus. The local freights were routed around Pt. Defiance to Nisually and back towards Tacoma to do their work, this adds about 25 miles each way.

As George said, the big issue for Sounder was connecting the two lines that were separated by a mile (actually about 1/3 mile, the connecting points will be about a mile apart). Sound Transit has rebuilt the Prairie line completely, I mean the even removed subroadbed. The big question is why did they do all this work before having an approved and funded plan to build the connection?