YOU CAN'T SEE it in the multi- colored route map of America's capital-city subway system, but running alongside the proposed Green Line to the poorest neighborhoods is a pale line from the White House. After cutting out any further federal money for Metro construction in the budget proposal just sent to Congress, the administration is telling its local and state government partners how they could piece together a section of Green Line anyway -- even if it means scrapping decades of politically and financially delicate agreements made with every past administration since President Eisenhower's.

All they have to do, says Mr. Reagan's mass transit chief, Ralph L. Stanley, is use what money Metro has on hand -- or is entitled to but is being held up from past years by Mr. Stanley -- to build the central section of the Green Line from U Street to Anacostia and to add a little Red Line out to Wheaton. This way, you see, people might not notice that the Reagan administration is trying to cut off funds that -- under a schedule worked out last year by all parties -- would link the poorest and most transit-dependent people in the region with jobs while providing long-awaited service to residents who have been paying into to the subway system all along and who have been waiting patiently for their share of service in Maryland and Virginia.

If Metro doesn't like this, Mr. Stanley suggests, the various governments could whip up their own plan to make up any difference with additional state, county and city funds and maybe some private money too. But it isn't as if these participating governments have been ignoring their financial responsibilities. The region's taxpayers have been paying up all along -- and that share may have to increase in the future, as determined by Congress and the White House. Dedicated sources of state and local revenue can -- and should be -- committed to that end.

But at this point, members of Congress from both parties recognize the fiscal foolishness of wrecking long- planned completion of the national capital's 103-mile subway system just as it is about to respond to its most patient partners in the agreement. And Capitol Hill is where good sense can prevail to complete the project frugally and equitably.