WITH UNUSUAL SPEED and by an impressively lopsided margin, the Senate has placed Congress squarely behind completion of the Metro subway system. Legislation authorizing the necessary federal matching funds for construction sailed through the Senate the other day, 66 to 23, with important blessings from the four most directly interested members: Virginia's Harry F. Byrd Jr. and John W. Warner, and Maryland's Charles McC. Mathias Jr. and Paul S. Sarbanes. Major credit for shepherding the bill through Congress goes to its House sponsors, Reps. Herbert E. Harris II of Virginia and Fortney H. (Pete) Stark of California. Now -- in no uncertain terms -- it is up to the legislatures in Maryland and Virginia to carry out their part of the bargain.

Lest there linger doubts in Richmond or Annapolis about the fiscal soundness of this transportation venture, here is comment from Sen. Byrd, who is not exactly Mr. Big Spender on Capitol Hill: he says he favors "completion of the system as part of a commitment to states and localities," and he calls Metro "an important part of the government's transporation system." This endorsement, along with Sen. Warner's enthusiastic support, should serve as a cue for Gov. John N. Dalton to respond intelligently with a sound state proposal for financing transporation throughout Virginia.

With a little ingenuity and leadership, Mr. Dalton could pave the way for a popular package of transportation improvements sought by legislators from various parts of the state. His administration already has suggested treating Metro construction the way it treats bus purchases and highway projects; the state, using some of its federal transporation funds, would assume 95 percent of the remaining Virginia cost of Metro construction. This makes sense, and should further smooth the way for Northern Virginia legislators to support an increase in the state gasoline tax for transportation projects, in exchange for permission to raise local taxes to cover Metro operating expenses.

In nearby Virginia as well as in neighboring Maryland counties, elected leaders do recognize the obligations of their local governments to meet the federal requirements of a "stable and reliable source of revenue" to help meet Metro costs. They believe, too, that decisions on how or whether to raise their own taxes for that purpose should rest with their constituents. These are local options that governors and legislators in both capitals can reasonably allow -- and should.