WHILE CONGRESS WORKS out dollar details of another federal share in the historical commitment to complete a subway system in the national capital region, Metro and local officials have been making important progress on a key line. The two moves go hand in hand: with enough support in both houses of Congress for adequate construction money,

Metro (having settled all longstanding major disputes over the route of its Green Line) could proceed to build, promptly and efficiently. If Congress recognizes the ultimate fiscal sense of approving the amount agreed to by the House -- $250 million -- Metro's participating governments could, and should, concentrate on strengthening regional sources of operating revenue.

That's how it is supposed to have been all along, and how all parties to this project -- Congress, past administrations and local and state governments -- have committed support to construction of the full subway network. It also was how Metro had been proceeding with this administration -- until mass transit chief Ralph L. Stanley told a Senate subcommittee last month that a 25 percent cut in the amount that had been agreed to "in principle" before then would be just fine.

In the meantime, and in spite of the adminstration's reneging, the Metro board has cleared the way for the Green Line, including a long-delayed section from the Fort Totten rail station in Northeast Washington to the Avondale area of Prince George's County. Like so many other disputes that developed over this route, including a complicated court case, settlement came only after careful negotiating between District and Prince George's officials, including members of Congress.

Now Congress can recognize and answer this good-faith effort by approving the federal share that can keep construction on the most efficient timetable possible.