FROM OUR intrepid correspondent deep below the surface of famed Connecticut Avenue comes official word that the shortest distance between a circle (Dupont) and a busy uptown complex (Van Ness: the center, the street and the university campus) is a Red Line. Metro is ready to roll with another section of subway system come December. That's a good six months earlier than scheduled. Metro management claims the eariler opening could actually cut -- repeat, cut -- the area's transit losses by more than $40,000 in the fiscal year beginning Oct. l.

Past openings of subway-route sections have added millions of dollars to operating costs. But Metro General Manager Richard S. Page says not so this time: savings would result because bus service along the avenue could be reduced. This, he estimates, would attract 28,000 daily riders into the subway system, and Metro would keep the change -- because combined bus-to-subway fares for commuters would be higher.

But even though the move is recommended by Mr. Page and his staff, it remains unofficial, with a final decision still to come from the Metro board. The changes does require some rejiggering both by Metro and the District government, though, to work well. For example, since no new subway cars will be available for some time, some trains will have to be shortened to keep the same intervals between trains. That will mean a little more crowding during rush hours. Also, to effect any savings, the city must have the political courage to reduce substantially the Connecticut Avenue bus service that the submway would replace.

Still, all good sense points to the early opening. The corridor is heavily traveled, not only by residents of upper Northwest but by tourists (to the zoo) and students (to UDC). When tracks, stations and riders are ready to go, Metro should be, too.