METRO HAD A BIRTHDAY last Sunday that shouldn't be allowed to pass without notice. Metro's first year of operations was auspicious. Its abbreviated route - essentially a shuttle across downtown Washington - attracted far more riders than anyone foresaw. It opened a new lunch hour world for thousands of persons who work in the downtown area. And it ran trains that were generally on time, clean and, like its stations, remarkably safe.

Once that is said, however, the reality of Metro's second year must be faced. This could be the make or break year. For the in the next 12 months Metro faces not only the continuing problem of financing but its first real test as a transportation system for commuters. In July, if all goes as planned, 12 more miles of track will open, including the tunnel under the river, and the process of diverting some of Northern Virginia's commuters from buses and cars to the subway will begin. Only then will it be possible to make a realistic judgment on whether Metro will eventually speed up commuting, relieve rush hour traffic and reduce air pollution.

While those are the standards by which Metro's second year is likely to be judged, the experience in downtown Washington suggests to us that there are two additional tests. One is whether the arms of the subway will open up suburban areas to new uses as its first leg has done in downtown Washington - for this has clearly improved the quality of life in the city. The second is whether Northern Virginians will take to the subway the way much of downtown Washington has done. Our impression is that Metro has been winning supporters, not just riders. If the same is true when its tracks cross the river, political problems now threatening the long-term financing of the system suddenly will become much easier to solve.