If you don’t mind losing I-90 for car use, you could just repurpose the right of way. Otherwise, you just need to build another I-90 for HSR, well, it wouldn’t need to be as wide, you probably could do more tunneling and viaducting through the mountains so it doesn’t slow down much like the real I-90 does. But in Seattle, I don’t think there is room for another new right of way, so you have to tunnel or somehow run it down the middle of the freeway (and forget about lake Washington, you would need a new floating bridge unless you could stomach the slowness of going around.

All can be solved with lots of money (and the space issues can be solved with even more money).