THE MANAGERS of Metro have approved a proposal to extend rail service past midnight on weekends -- a good idea though not the best test of it. The point of the change, advocated by a substantial number of elected officials around the region, is to serve tourists and residents who depend on public transportation and who have to cut short their evening activities in time to catch the last trains back.

Many supporters of the change sought to extend Friday and Saturday evening service to 2 a.m., when night spots close. But the approved proposal is for an eight-month trial period of service only to 1 a.m. After six months, Metro's directors say they will examine the program and if it is deemed a success, will consider extending service to 2 a.m.

That may seem to be a logical compromise, but a number of business owners who operate downtown gathering places believe that the 1-to-2-a.m. hour might attract even more riders than 12 to 1. They worry that an ill-publicized trial run of a one-hour extension may fail to meet the Metro directors' ridership expectations -- and that as a result the second hour will not be tested. It would have been better to start by examining late-night ridership patterns for the two hours.

At least Metro hasn't closed the doors on a second hour of service. The plan is an improvement and does promote transit travel. Metrorail's current midnight shutdown on Fridays and Saturdays is the earliest among the country's major transit systems. Leaders of cities with night-owl service note that sports events, late movies and live second shows generate needs for transportation. Also, some patrons who stay until closing time at suburban as well as downtown taverns might be much better off on the rails than on the roads.

Regardless of the results of Metro's new plan, some trial of the two-hour extension ought to be conducted, not only to find out who would use the system but also how much in additional tax receipts might be generated from restaurants and entertainment complexes. Metro should give this option a full go.