I don't know what bringing back the SP&S is going to accomplish; you still have to deal with bottlenecks at Spokane to the north and Pasco to the south, and it isn't as though the ex-NP is itself a problem.

It would seem that a better solution is for the BNSF to buy back the Milwaukee right-of-way they had purchased but then sold off to the State of Washington, build it as a state-of-the-art mainline (even going so far as to buying up the right-of-way through the CWU campus in Ellensburg, unless a new ROW can be routed around it), and rebuilding the yard in Othello as a storage yard (in the event something happens west, there's a place to park trains other than on the mainline).

This would not only add capacity, it'd also shorten the distance from Spokane to Seattle, and alleviate congestion in Pasco. If BNSF really wanted to they could continue east towards Missoula, but parts of that ROW has been turned into highways, along with a very popular (and privately owned) mountain bike trail.