IN THE ADS promoting the new night and Saturday subway service that started this week, there's a slogan that we like: "Metro's New Washington . . . It's Getting Better All the Time." It is a new Washington - a Greater Washington, in the regional sense - that is coming together in new ways as the rapid rail system expands.

Downtown, we're getting used to the lunch-hour-rush-hour, but one still notices the Pentagon brass and suburban homebodies riding in for get-togethers along Connecticut Avenue, on Capitol Hill or in Rosslyn. And now that the cars are rolling until midnight, there's just a little less madness in the rush to get home; carless office-workers, students, moonlighters, diners and theater-gosers feel freer to hang around. And many regular plane and train travellers are content not to have to leave their cars in costly lots now that subway service doesn't stop at 8 p.m.

Starting today, Saturdays will be a little different, too - with new freedom for shoppers to shuttle between downtown and suburban stores and for families to revisit the Smithsonian or the National Gallery, places that drivers in search of parking spaces gave up long ago as hopeless on weekends. Every now and then a Monday night will be a little special underground as well - beginning next week when the Redskins play Dallas at RFK. Will some of the old tailgate gangs in the parking lot start circling their wagons around suburban subway stops? And next fall, with the planned start of seven-day subway service, the rites of Sunday - for church-goers, sports fans and family-outers - will change as well.

IT goes without saying that the night and weekend service costs money, and that this is but an experiment. In relation to the total net subsidy for Metro-rail service, the estimated increase for nighttime, Saturday and Sunday service comes to something under 5 percent. In the long run, however, the test will not be solely at the farebox, though new service may well generate additional activities and store operations that in turn could generate more riders and revenue. There may also be savings from cutbacks and changes in bus service. And as rail service continues to expand, it may win many more friends who will see the value of a fulltime, dependable subway system.