A LITTLE MORE than a month ago, when Metro's board of directors was not yet ready to agree on anyone for the top job of general manager, the members asked Carmen Turner, an assistant general manager, to run things for a while. It was understood by all that she was not a candidate for the job, that it was a matter of paring a list of five contenders, including two other assistant general managers. Then the other day, the board members looked around again, discovered an able candidate already doing the job and agreed that Mrs. Turner should have it. And all evidence indicates they have chosen well.

Mrs. Turner is not new to transportation, government, management or Metro. Her experience in the federal Urban Mass Transportation Administration provided a natural introduction to Metro, where she has been on the staff for six years. Most important, Mrs. Turner enjoys the respect of board members from the District, Maryland and Virginia--members who have been increasingly at odds over Metro's constant financially and politically complex issues. On more than a few occasions, Mrs. Turner's thoughtful, conciliatory approach has been credited by members with having settled potential deadlocks.

As always, the new general manager's plate is full. Subway construction still has miles to go and a critical Green Line is still to come, the sooner the better. Too many buses are ready for that great garage in the sky, and fares are already that high. Mrs. Turner is fully aware too of the dangers of letting service slip; convenience is key to ridership. And, of course, there is the matter of the tab for all this: somewhere in the future, a dependable regional tax structure has to come into being, to ease and equalize the financial contributions of all the local jurisdictions.

In the immediate future--Monday, to get specific --there is the annual test of the subway and its ability to help Americans enjoy the Fourth on the Mall. Happy holiday, Mrs. Turner, and good luck in a grueling job.