IF THE OPENING DAYS are any guide, Metro's blue line is going to be a smashing success. True, there were equipment breakdowns and the inevitable delays, confusion over fares, long lines and other difficulties. But the trains and stations are - as promised - clean, cool and quiet. And the passengers, so far at least, have been eager, friendly and understanding, as well as exceptionally numerous.

Metro, of course, is still in the novelty stage. Thousands of the blue line's early passengers were just out for the ride - to National Airport, the Stadium or around the system. Its facination as a tourist attraction will continue, no doubt - the escalator ride at Rosslyn and the view from the airport station are worth the price of admission. But in the next few days, the real business of Metro begins. Its job is to move people around the area faster and better than ever before, and it has already provided a whiff of what can be possible if both its staff and its owners grasp the opportunities that are now open.

For Metro's staff, this trains to run on reliable schedules, maintaining other equipment so that it works when it is supposed to and keeping the trains and stations clean and safe. For Metro's owners - all of us who are paying for it with tax dollars - the tests are different. They are how much we use it and how we elet it change the pattern of life in this urban area.

No one should anticipate a substantial change in automoble traffic across the Potomac bridges just because the Northern Virignia stations are now open. It will take time - and, probably, and end to free parking for government employees coupled with increased taxes on other downtown parking - for Metro to make of a dent in automobile congestion. But it should not take much time for other changes commute by public transportation and that it will begin to incorporate the Virginians and that it will begin to incorporate the Virginians who already commute by public transportation and that it will begin to incorporate the Virginia side of the Potomac into the area's urban center. Aquick trip over to Rosslyn from the District; for Norther Virginians, luch and shopping downtown - these are now real possibilities for thousands who previously would not knwo exactly what Metro is going to do to this community's habits and traffic patterns. But we've seen enough of the blue line to be confident that the subway system, true to its won adversiting, is going to produce a "new Washington" - not only significantly different from the one we now know, but also, in many respects, substantially improved.