WASHINGTON'S SUBWAY SYSTEM isn't a toy any more. As of tomorrow morning, the subway becomes a significant, working part of the region's mass-transportation system. This is not to say that the little "red line" that's been running between 8th Street and Rhode Island Avenue NE and Dupont Circle hasn't been useful - as far as it goes. A smash hit since it opened more than a year ago, the first, short segment became the daily way-to-go for a solid core of some 20,000 commuters. But now, with the opening of the "blue line" from RFK Stadium across downtown and on to National Airport, the local transportation revolution can begin.

Now it is true that stations at National Airport and the stadium aren't placed as conveniently as they might have been, and neighborhoods around the 17 new stations aren't exactly thrilled by the prospect of automobile commuters cruising for parking. For a while, too, subway riders will be unfamiliar with the fares, farecards and transfer systems (though we suspect that they will prove easier to use than to read about).

But Metro is on the move. Virginia and downtown will now be only a few smooth minutes apart. Offices, schools, stores and recreation centers on both sides of the Potomac will be within easy reach of each other. And for many a longtime automobile commuter, the rush-hour jam at the bridge can become a thing of the past. Better still, sometime in November (if Metro's present construction schedule holds), the system will connect with Maryland, too. The red line will move on out beyond 8th and Rhode Island NE through Takoma Park and on to Silver Spring.

Just how all of this is going to affect traffic congestion in general remains to be seen. As staff writer Douglas B. Feaver reported in this newspaper last week, the experience of other cities has been tha the people who first make use of new public-transit facilities are already mass-transit customers: "It takes time, good service and convenience to build patronage among those who are accustomed to driving their cars."

Over the next two months, though, more than 500 bus routes will be changed or eliminated to coordinate with the rail system and to reduce duplication in services. So at least some slight reduction in bus congestion at certain spots ought to be noticeable.

Beyond that, just as the opening of the Capital Beltway began changing the living habits and work patterns of the region, so will Metro. According to a study by the Metropolitan Washington Council of Governments, the biggest potential impact on automobile traffic may come later in the decade, when Metro reaches the beltway. If the presently proposed 100-mile rail system is completed, nearly all of the residents in the District, Arlington and Alexandria, along with those in large areas of Montgomery County, will be within 45 minutes by public transit of half the region's jobs.

So bring on that blue line, with farecards at the ready. As Metro's been saying, "You're one of the owners. Use it. Enjoy it."