WHATEVER THOUGHTS the Reagan administration may have had about brutally pruning the Metro subway system must have been transitory, because Transportation Secretary Drew Lewis has just delivered an encouraging word: He sees not only light at the end of the tunnel, but also tracks.

It's still a little early to dash out for the 8:02 from Vienna or Anacostia, but patience should have its rewards: Secretary Lewis says the administration is "firmly committeed" to a 75-mile system. True, that's not exactly an ironclad pledge to go the full 101 miles as scheduled; but on this point, too, Mr. Lewis brings fair-to-good tidings: "I anticipate that the additional miles will be completed. We are waiting on the local governments to see if their funds will be available for that."

Reading between the lines, this means that the system can continue to fall into place in a reasonable and frugal manner as its strong bipartisan supporters have thoughtfully and carefully proposed. A 75-mile system would include the 37 miles already in operation plus major segments of every planned line. Secretary Lewis is proposing that federal funds for construction be limited to $275 million a year, which is the same amount the Carter administration had approved in recent years. Before leaving office, Mr. Carter had agreed to a new level of $350 million, and Metro will continue to seek that amount with good reason: the sooner the system is completed, the less it is likely to cost.

For now, though, the administration has chosen a sound, cost-conscious and thoroughly justifiable course. In that spirit, the state and local governments, too, will have to keep on delivering their end of this carefully fashioned bargain. That's no small task, but to those who have paid-in-waiting for their neighborhood's turn on the trains, completion of the outlaying tracks is not only a vital service but a matter of fairness.