We usually call Sounder “regional rail” or “commuter rail”, but both terms are ambiguous. Other possible terms like “metropolitan rail”, “local rail”, “express rail”, or “rapid transit” are also ambiguous. That leaves me at a loss with what to call Sounder or RER that’s not ambiguous. The same problem exists with Cascades.
“Regional rail” implies an area with multiple political entities. Both Sounder and Cascades are called “regional rail” but are at different scales. Sounder goes out 50 miles like Caltrain, connecting suburbs and cities within a multipolar metropolitan area. Cascades is 500 miles long, connecting multiple nearby metropolitan areas. High-speed rail plays a similar role. So we need distinct words for Sounder-type networks and Cascades-type networks.
“Metropolitan rail” implies the city and suburbs within an metropolitan area. This sounds like Sounder, except the term “metropolitan” has been monopolized by subways. Subways are shorter, have closer stop spacing, and higher frequency. Paris has both RER and metro, making this distinction between them.
“Commuter rail” originally meant riding on a “communtation ticket”, or multi-trip discount ticket like a 10-pack. This has led to a bifurcation, with some commuter rails running full time and others peak-only Caltrain and PATH run full time bidirectionally, so they’re as good for a weekend trip to the museum as a weekday trip to the office. They’re intended to capture the bulk of trips in their area to minimize driving, both work trips and other trips. Other commuter rail network are peak only, serving only 9-5 downtown office workers, and there’s resistance to expanding them to other uses.
“Rapid transit” to me means faster than a regular bus, so grade-separated, wider stop spacing, and higher frequency. Others use the term specifically for third-rail or heavy-rail metros.
Some people use “S-Bahn” or “RER” generically to refer to this mid-level service, but most Americans have never heard of those and don’t know what they are.
So is there a unique and unambiguous way to refer to Sounder, RER, S-Bahn type rail? Something that gets at the four-way distinction between Link, Sounder, Cascades, and intercity lines? Because “regional” is too ambiguous.