Places like Gresham to Forest grove are better for the Portland tram network, they are just part of the one metro area, they are not different metro areas like Seattle and Portland.
You do want some local service at some in-fill stations though between the major destinations, even run express and local service, but as long as you can park your car at a station, having a station that the more suburban/rural parts slightly off the beaten trackway can go to still makes it accessible to more people in the State.
For the East/West corridor slower speed trains at the same stations allowing transfers would be preferable (cheaper and quicker to build, with minimal speed impact) to high speed rail.
You can run them on the same tracks with the same stock. You have e.g. the 9:00 express train depart from Portland serving a subset of the stations between it and Seattle and maybe the 9:15 departs and serves all of them followed by another express at 9:45. Each stop shouldn’t add more than 2 or 3 minutes between terminuses, if the service stops for a generous amount of time at each one.
And then yeah, you have your regional transportation agencies to serving the metro areas along the way more comprehensively.