I think it likely that engineering studies for a new tunnel at Stampede exist somewhere in BNSF, I'd even bet that they've been thoroughly reviewed in a decision the enlarge the current bore. While a new bore might be ideal, it would not be cheap or quick, given the active geology of the Cascades, faults, and the like. Could it be that there are substaintial problems that have been discovered in any alignment of a new bore? Also, TBMs are very expensive items; that cost alone could far exceed the cost of fixing up the current tunnel. While Stampede does have heavy curvature, especially btween Lester and Stampede, Stevens does also, between skykomish and Berne (It laso has some very narrow cuts). The old MILW also had heavy curvature. Bottom line: the Cascades are an interesting railroad challenge, even today.
Now, about those bricks (How the NP got them cheap is another quite interesting tale). Why don't they sell them at, say, $50 each to foamers, FAS Stampede, and no hucking at passing double stacks, either.
OK, now that I've had a meow, I can't help but wonder when they'll fully signal the main between Kennewick and Auburn.