Credit Transportation Secretary Elizabeth Dole with an impressive demonstration of good faith -- and the money to back it -- in Metro's rapid transit system for the nation's capital. Yesterday the Reagan administration lifted its limit of 76.4 miles on subway construction by agreeing to turn over money for vital extensions of the network into Prince George's County and Alexandria. The move is significant not only in terms of the additional miles of track, but also as a tangible measure of this administration's support for a model transportation system that state and local governments are deeply committed to complete and operate.

Had it not been for the continued monitoring of Metro's progress by Secretary Dole, the subway system might have been thrown into deep and expensive troubles. The decades-old, unique regional agreements that have made everything possible up to now have all been predicated on the eventual orderly completion of a 103-mile system. The last 14 miles of that are still subject to federal consideration at some later date, but yesterday's breakthrough allows Metro construction to continue without costly delays and without shortchanging any particular area of patient would-be riders. The idea is still to spread available money as evenly as possible, and negotiations so far are doing that.

Key support for the latest federal understanding also came from Ralph L. Stanley, the administration's mass transit chief, who has been following Metro's operations closely through each stage. With the additional assistance, Metro can begin work on a Green Line station at West Hyattsville and a Yellow Line spur to a Van Dorn Street stop in Alexandria's West End -- both scheduled to open in the early 1990s. Already in the offing were extension of the Orange Line to Vienna next year and building the Green Line between Anacostia and Northwest Washington and a Red Line connection to Wheaton.

So Metro is getting there, mile by mile. And this is all the more a signal to the state and local governments that their commitments won't end when all the track is down. On the contrary, they will mount, because the more system there is, the more system there will be to run -- and the more challenging the fare-and-subsidy structure will become. It will take some courageous doing, but dedicated sources of revenue will have to be found and agreed to.