THOUGH IT DOESN'T generate the same public excitement as the opening of a new stetch of subway track, there is cause for cheer in the news that a financial milestone for Metro seems to have been achieved this week. After some hard negotiations between federal and local officials, there is a tentative agreement on one of the toughest money problems that has bee n blocking completion of the planned 100-mile system. Credit goes to Transportation Secretary Brock Adams, who worked out the accord with the Metro board.

At issue was how to cover the debt on $1 billion in construction bonds that were sold by Metro. Originally-at least in the minds of dreamers-those bonds were to be repaid from fares. Instead, Metro and the federal government up to now have been scraping up semiannual interest payments through a series of crash-crisis combinations of congressional appropriations and the use of investment income Metro has earned on unspend construction dollars. But that investment money has been running out, as has the patience of officials every time this happens.

Under the agreement, the federal government would pay two-thirds of the interest and principal.

Metro, with help from local or state governments, would pay the remaining third. With such an accord, says Secretary Adams, he is ready to recommend $275 million in federal funds for construction for fiscal 1979 and the same amount for the following fiscal year. That's not as much as Metro officials had hoped for, but it's certainly more than the federal geovernment had offered up to now.

Before dancing in the streets, however, there are a few matters to be nailed down-not the least of which is securing final approval of the White House's Office of Management and Budget and then of Congress. Mr. Adams says he's satisfied that this can be achieved; we'll se.