IF ALL THE ups, downs, approvals, denials and provisional acceptances of plans for construction of Greater Washington's subway system have left you uncertain about exactly what will be built and when, you're in good company: Behind those dotted route lines on the Metro map are a lot of question marks. Still, there is noteworthy if limited progress this week that should keep more tracks and stations coming for a while. The Reagan administration has agreed in principle to the construction plans that were negotiated and submitted by the region's Metro board members. That's not a formal federal go- ahead for additional miles of track, administration officials are quick to note; but if costs and schedules can be maintained as presented, prospects for 89.5 miles of the full network look good.

Transportation Secretary Elizabeth Hanford Dole, whose knowledge both of the history of this project and of its national as well as regional significance has been critical to Metro's orderly construction. It is also a challenge to the state and local governments that have been working together with the federal government for decades to complete the system as originally envisioned. The ultimate goal of a 103-mile system is still quite alive; what has to come alive as well is a strong and formal commitment to come up with regional financing for what promise to be the heavy operating costs in the future.

So far this delicate work has been handled well by Metro. Both Metro General Manager Carmen Turner and current board chairman Gladys W. Mack enjoy widespread respect and recognize the necessity of keeping costs down and efficiency up. But they and anybody else who wants the full 103-mile system to be completed have got toome state and local money where their hopes are.