WHEN THE TRACKS are ready and the stations seem set, can subway service be far beyond? Yes -- which is a matter of no small concern to many would-be riders in Alexandria and Fairfax County. They, like all taxpayers downtown and around the region, have paid goodly sums for the coming of Metro -- and thus have more than a casual desire for a piece of the traveling action. But with Metro facing a shortage of subway cars, it is not a matter of course that trains will come to those who pay and wait. The Metro board could decide that existing service on the current routes should not be diminished and that any extension of the subway should be postponed until new cars are in hand.

Unfair. While it's true that riders along the existing routes may have to endure some schedule changes and shorter trains, those who were led to believe that their local subway facilities would open soon should be treated to service as promptly as their stations can open. That's how the board decided to open service on the Red Line to Van Ness Street NW last year, and similar sacrifices should be made to extend similar favors beyond the Beltway in Virginia.

There is a dividend in such a decision, too: there is good reason to believe that the eagerness of people to support -- make that, chip in for -- the Metro rail system is highest among those who can use it easily and often. There is just as good reason to believe that those who might stare at ready but unopened facilities because of a car shortage may run out of patience, if not money.

Obviously there is a limit to how far a given number of cars can be stretched. But the same holds true for patience.