AS ALEXANDRIANS and other bus-plus-subway commuters surely noticed, Metro's color-coordinators are talking about adding yellow to their list of subway lines soon -- but without taking it beyond National Airport. As recommended by the Metro staff, service between Gallery Place and the airport would begin in April, six months earlier than previously planned. While the opening of any new subway service merits a cheer, those Virginians who have been staring at ready but unopened subway facilities running from the airport into Fairfax County may well wonder what it takes besides patience to get their stations in service.

The last time we wondered in this space, Metro's response was that it has a serious shortage of subway cars, and that existing service would be too severely shortchanged if any more service were added. This was what Metro said before it finally opened service on the Red Line to Van Ness Street NW last year. For better or maybe even slightly worse, riders seem to have survived the dilution of cars on each run since then.

So what's the rationale behind opening another downtown leg of subway now, before anything else? Metro notes that the new Gallery Place-to-airport service should attract many who now ride the Blue Line to a faster, more direct route across downtown to the Pentagon. That would ease strains on the Blue Line and thus not require a serious stretching of the limited supply of subway cars.

Maybe, maybe not. But it may prove car-wise and future-foolish to keep putting off the many would-be riders in Alexandria and Fairfax whose considerable taxes and support have been critical to the birth and growth of the subway system all along. They need some tangible evidence that their investment is a good one -- and there's no better reward than a train in their home station.