THE NAIL-BITING at Metro over how and when the Carter administration would come to the financial aid of this region's subway system is over - having given way to head-scratching and brow-furrowing as local officials assess the response last week from Secretary of Transportation Brock Adams. The gist of his reply is that yes, Metro can have some more federal money to meets its immediate needs and yes, there should be more federal participation in a current study of unbuilt subway lines. While this falls short of what Metro officials consider enough, it does buy time to concentrate on addressing the system's long-range planning and financing.

Of greatest immediate concern to federal as well local officials are the interest payments for $1 billion in revenue bonds that were sold to pay for Metro rail construction. Metro officials have been pushing for federal assistance for 80 per cent of the interest; Mr. Adams has now offered to pay 80 per cent of next fiscal year's interest. But that leaves Metro with a $12-million share due July 1.

And it is Metro General Manager Theodore C. Lutz's sad duty to report that the system doesn't have the $12 million. If the payment isn't met, Metro will be technically in default on the bonds, which are federally guaranteed. So that leaves an expensive matter still to be resolved. If the local jurisdictions expect continued federal assistance, there will have to be some sort of regional tax or taxes to cover the local share in the future. With encouragement from the administration, this response could - and should - be linked to some strengthening of regional cooperation and leadership, as suggested by Atlee E. Shidler, president of the Washington Center for Metropolitan Studies, in excerpts of a letter that is published on this page For the Record.

In turn, the financial pressures are linked to a current "alternatives analysis" - a restudy of subway lines beyond the 60 miles either under construction or under contract. The purpose is to decide whether subway, trolley, bus, highway or anything else might be substituted.

Just how much additional federal participation Mr. Adams envisions in the planning or policymaking ranks isn't clear. But his proposal that a 4.5-mile Silver Spring-Wheaton-Glenmont line be restudied doesn't seem as frightful to us as it does to Montgomery County Executive James P. Gleason. While his anxiety over the line's fate is understandable, a favorable reassessment would strengthen the argument for federal financial support for it.

Moreover, the designation by Mr. Adams of a Department of Transportation official to help work on the Glenmont line study doesn't have to mean federal interference. When federal "interference" produces the money and results sought by local jurisdictions, we like to think of it in more constructive terms, as "federal participation," let us say, or "federal aid."