ONCE AGAIN, White House and Metro representatives have whipped out the calculators and subway maps for another in their series of dollars-for-miles negotiations. Just when everyone thought the administration had stopped tinkering with the routes, along comes a White House Proposal for a new financial arrangement containing some large disappointments for would-be riders, particularly those awaiting more service in Maryland.

Many proposals have come and gone, and this surely is not the last word from the White House - nor should it be. What it would do is modify a three-year schedule Metro had put together, leaving out a major segment of construction on the Glenmont line in Montgomery County as well as top-priority routes sought by Prince George's. That, given the delicate local politics involved in financing Metro, is a bad idea. Metro officials have always strained to include a little something for each jurisdiction, for already Prince George's and Fairfax are hungry for subway service. Anyway, why is the federal government still fiddling around with Metro's construction scheduling plans?

The answer goes back to the last Metro money minuet a month ago. The White House was threatening to cut off money needed immediately for 90 new subway cars. That threat was properly withdrawn, but the administration's reason for pestering Metro did not disappear. The federal government is still demanding assurances of Metro's share of debt coverage on $1 billion in bonds.

That's understandable, since the contributing state and local governments have been less than swift in making a formal, specific financial commitment, including some system of taxes dedicated to the transportation system. But because the state legislatures in Annapolis and Richmond have yet to do their part, and because some jurisdictions would require voter referedums first, the local governments cannot sign a complete local guarantee now. What these officials should do is draw up the most specific agreement possible at this point and sign it. In turn, the administration should get away from those route maps and let Metro coordinate the construction schedule.