> Trains already mass over/through both mountain ranges.

They do, but not on routes that could support a high-speed train for the most part. Though OP has actually missed the most difficult mountains, which are the steep, uneroded peaks of the Cascades and Sierra Nevada. To this day, only a couple of rail lines cross them, and all of those involve some pretty curvy paths except for the ones through the Columbia Gorge.

It's obviously not impossible, but it's not as simple as using existing rail lines either.