The math is very clear. Below about 25,000 people per linear kilometer of lines, they require massive subsidies and never really pay off. Eugene to Vancouver BC is <12,000 people per linear kilometer. Honestly I would love to be wrong.
Yet the distance between Seattle and Portland is the same as between Frankfurt and Munich.
There's simply not enough traffic to justify a train. Another lane on I-5 would help a lot more.
That rarely works out how you would think:
They are terrible, and always will be. It's simple math: buses have to either have long intervals, or be horribly wasteful (and thus too expensive).
At the moment, the most realistic solution is to just wait 10 years for the self-driving cars to mature and then build infrastructure around them.
Something in between that and buses might be very efficient and practical, especially for things like work commutes, visits to the supermarket and the like. Having say 6 or 8 seats and picking up multiple parties who are heading in the same direction or going to roughly the same destination. It would likely be quicker than waiting for a single trip car and definitely cheaper. Additionally it would likely reduce the number of autonomous vehicles on the road and use less resources.
Extending Waymo to handle ALL possible situations autonomously (you put in "New York" in the address field and the car drives you there from Seattle) is probably a too big ask, but working within a pre-mapped area on semi-fixed routes seems plausible.
Induced demand is a sign of success: it allows more people to move comfortably between points, and it increases the overall economic activity by inducing more trips.
The only reasonable part of that argument is that lane expansion doesn't increase the average _speed_. And this is indeed true. Instead of lane expansion we ideally should just increase the interconnectivity by building new roads.
In the case of the Puget Sound, I-605 that would bypass I-405 bottleneck is sorely needed.
By contrast, all the other standard HSR candidates anchor regions that involve at least one, and for all but the Midwest two, cities in the top 10, with many more in the top 50.
Show me a similar area with fewer than 12,000 people per linear kilometer that has HSR and I will be impressed and take notes. I don't know of anywhere on earth that has fast trains and that few people.
There are 10 million people in Portland, Seattle and Vancouver metro areas, and the distance between the three is about 500 km. That would be more than enough density for high speed train in any part of the world. Spain's major population centres are a lot more geographically spread out.
I'm not sure I'd start with high speed rail in this corridor. The focus should be on high-frequency, high-reliability fast trains that may not hit the true HSR speeds, but at e.g. 200 kmh would be plenty fast enough for now. But the argument that "the population density isn't there" for HSR is absurd.