The better example of an abandoned route through the Cascades is the old railway route through Snoqualmie pass, which connects the metro Seattle area to the east side of the mountains. It's now a state park, and you can hike or bike from one side to the other.

As technology improved, a longer tunnel was built lower down the pass to decrease the elevation gain for trains, and the old route became obsolete. I suspect that this is not a unique story, and the question would be whether the old railway routes are still safely navigable.