IF GASOLINE STATIONS around town really do shut down on Sundays, that will be yet another reason for starting to run the subways a seventh day each week -- a proposal we've supported all along. Right now officials in the region are considering whether to begin Sunday subway service this fall. If the public's response to the addition of evening and Saturday service is any indication, the Sunday trains will be popular. For less than one percent of the rail system's operating subsidy, Metro's people tell us, the trains could roll from 10 a.m. to 6 p.m. every Sunday.

Those hours would help those going to church, as well as shoppers, train and plane travelers, museumgoers and people attending events at the D.C. Armory and RFK Stadium -- including the Sunday games of the Diplomats and the Redskins. In warmer weather, Sunday service to Arlington Cemetery could serve the tourists, too. Besides, the subway has proven to be more than a commuter transit system -- it is bringing downtown and suburbs closer together in constructive ways.

Consider, too, the heroic role of the subway system during heavy snowstorms, such as the one we're still digging out from this morning. Though there weren't enough people on duty to provide complete Metro transportation on the holiday, the existence of an underground way to get around -- as well as to cross the Potomac -- was an enormous help after last week's big snow, and will doubtless play the same invaluable role this week. Indeed, the snow crises prove the point: When automobiles can't move around, their owners come quickly to recognize the value of Metro -- as they most certainly will if the developing energy pinch causes a Sunday shutdown of gasoline stations.

So we continue to urge the Metro board and the various local governments represented in the transit system to keep on expanding the subway's hours of operation, just as they have been expanding the areas of this region served by the system. A good start now would be to give that go-ahead for the eight-hour Sunday service on an experimental basis.