HAVING COME THIS far in building "America's Subway" in the capital city, can the federal/state/local partnership that has worked and paid together for all these decades find the money to go the last miles and then keep the system running? The answer is yes -- but the financial commitment to complete the job and capitalize on this investment will be anything but light. That's the most important message in some new and enormously useful findings made public yesterday by a special independent task force that has taken the hardest detailed look yet at transit finance in the Washington area. There are now some numbers for Congress, state and local jurisdictions to consider in a concerted move to complete the project frugally and equitably.

The study, conducted under the auspices of the Federal City Council with data and technical support from every participating jurisdiction as well as the federal Urban Mass Transportation Administration, is a look at several financial paths to the year 2000. That is when the 103-mile rail system is scheduled (and we use that word advisedly) to be completed. Besides cost of final rail construction, the study looks at the costs of rehabilitation and replacement -- keeping buses and subway cars rolling -- which will be both heavy and sudden when stock wears out.

Financial provisions for these costs -- a replacement account -- will have to be established and considered as part of the total bill for this project, no matter how they may be shared ultimately by the participating governments and Metro's riders. Similarly, operating costs will have to be considered, though they look to be relatively stable over the next 15 years.

The greatest uncertainty in any Metro math right now is the decision by the Reagan administration to cut out all federal spending for completion of the system as planned through the years by all parties. If left to stand, it would 1) throw orderly construction schedules into still more costly delays; 2) punish those areas of this region that waited the longest while paying their shares; and 3) end in one financial swoop a long history of federal commitment to an efficient transportation system in the capital.

All along, the taxpayers of this region have paid willingly and on time to keep the project going. Members of Congress from both parties have understood the federal role in this and have been prudent partners in the planning and financing. Now that process can continue with new and better information. But the state and regional governments will have to recognize their commitments with appropriate financial planning.