Ok, but if high speed rail from Seattle to/from those towns was added they wouldn't stay affordable for very long. In order for that not to happen lots of housing units would need to be added to those areas.

Those places already growing at a steady clip. They aren't very desirable though.

In high school, I lived in South Snohomish county (it was still just Bothell even though it wasn't in King Co). Within striking distance of Seattle, so it grew quickly. I had friends from Marysville and Darington however, they didn't like living in those places very much.