The Link light rail project won't even reach Everett until 2036. That's about 40 miles of track in the 40 years since the project was approved. How is this supposed to work?
But in general you're correct that projects requiring new right of way are very challenging and expensive. Especially high speed rail with limited turn radius. Since the article mentioned a low-usage rail link between the cities, I would prefer to see a campaign to increase ridership. Maybe you could incrementally upgrade segments of that route that could support high speed rail.
how could you improve those railways if they are not yours to modify on first place?
Even so, I'm guessing that there are options other than $50B of high speed track to improve the route. Maybe there's a few segments of track where a large fraction of the delays happen, and 30 miles of amtrak-owned track would really improve on-timliness. Just getting it from +hours to +minutes might draw some new users.
Having ridden those very commuter lines, I can say that is not an option here. They are limited to 40mph along those routes because they follow a very windy path along the shoreline. There have been derailments from trains going too fast.
When a train is going 400kph, you can’t share track.
Correct, you would be traveling at normal train speed on shared track. This is workable in some systems because you can build cheap (relatively) rural segments of high speed rail without needing new right of way through urban areas. Obviously wouldn't work everywhere, and maybe it's not the right solution here. But it is done some places, and places like California are considering building new blended systems [1.
[1] http://www.caltrain.com/projectsplans/CaltrainModernization/...
The political right has convinced a large portion of the US population that _any_ government spending is wasteful and unnecessary. Those people don't see government spending as an investment, they see it as theft.