AS MANY A SUBURBAN subway commuter-of-the-future is discovering, you needn't be a resident of Montgomery County to care what happens to Metro's plan to run its Red Line out beyond Silver Spring to Glenmont. On the contrary, thanks to some gobbledygook from the Department of Transportation and some political grandstanding by Montgomery County Executive James P. Gleason, the fate of this rail section has become an issue threatening the entire regional financing of the subway system.

The matter came to head in early summer, when Mr. Gleason - understandably frustrated by fuzzy answers from DOT about federal support for the 4.5 mile line - resorted to blackmail by withholdong his support for the release of $328 million earmarked for the construction of already programmed segments of the Metro system. This wrongheaded response in turn produced a local political crisis, bringing new Metro construction to a halt and imperiling the entire agreement to complete 60 miles of Metro's planned 100-mile network.

So it was with considerable interest that we read a report last week from Atlanta by staff writer Douglas B. Feaver noting that DOT, Mr. Gleason and Maryland and Metro officials had met there in an apparent attempt to reach some understanding on the Glenmont line. Though nothing concrete emerged, the parties apparently are taking a calmer look at the matter.

The federal position, according to Richard S. Page, administrator of the Urban Mass Transportation Administration, is that the government would consider allocating money for the line if the local jurisdictions will give it top priority. That, translated, seems to mean that 1) the region would have to agree that this line should be the No. 1 project beyond the 60-mile network; 2) local matching funds, including entitlements to federal interstate highway money, be committed to construction; and 3 the Metro board recommend building the line.

So DOT apparently wants all the justifications it can gather to make a case before the Office of Management and Budget that this or any other subway proposal is, as the jargon goes, "cost-effective." It seems unlikely that a federal "guarantee" is in the offing. Instead of continuing to hold back money, Mr. Gleason should concentrate on strengthening the case for the line. In the meantime, if DOT is seriously sensitive to this area's transportation needs, it should work harder to clarify its position in favorof the Glenmont line.