[SllX]

Ah, express and local service are terms of art used by at least Caltrain that I may or may not be wrongly assuming is more widespread (although another commenter below mentioned it’s used by the Shinkansen services in Japan as well which I think I knew but forgot). They have their “baby bullet” service which amounts to: these set of trains in particular skip a bunch of stations along the way, and the local service just serves all of them.

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.