> only a tiny fraction of commuters take rail to begin with
If you define "metro area" expansively, that's true, but rail mode share in the closer-in suburbs (Montgomery county, Arlington county) is decently high, and individual counties' contributions to WMATA's budget are commensurate with use. Not many people in Loudoun county use metro, but they also don't contribute very much, and nobody it Charles county does, but they don't contribute anything.
If you define "metro area" expansively, that's true, but rail mode share in the closer-in suburbs (Montgomery county, Arlington county) is decently high, and individual counties' contributions to WMATA's budget are commensurate with use. Not many people in Loudoun county use metro, but they also don't contribute very much, and nobody it Charles county does, but they don't contribute anything.