WE always expect the wonderful things will outlast us, that the good times will keep rolling. But this month two institutions threaten to bow out in the face of red-inked balance sheets: New York's Rockettes and the Washington-to-New Orleans sleeping car train, the Southern Crescent.

On stage in Rockefeller Center's giant Radio City Music Hall, the high-kicking, leggy chorus line of women was all glitter and polish that made a Washington high school class trip to Manhattan complete; backstage the atmosphere was family, recalls former Rockette Phyllis Wujiko who now sings and dances at Washington's Gaslight Club. Knitting bees helped pass the time between a strenuous show schedule: four shows a day, seven days a week, four weeks on and one week off. In the middle of the week, on payday, the Rockettes would gather for gossip at the Old Times restaurant or at the Assembly Steak House on 51st street where dancers paid half-price for food and drink.

It was a lack of revenue that led to the closing of Radio City Music Hall, and the tune may be the same for the passenger train called the Southern Crescent. Hauling freight turns a profit; transporting people doesn't. Southern Railway, which runs the premier north-south train, has lost millions each year keeping the Crescent alive. Now, depending on a regulatory agency decision, only the memories may remain, like the one that belongs to a Washingtonian who recalls a winter ride south at night, watching the frozen countryside etched sharp by moonlight as he lay in his berth.

It is April 1978, and cruel economics decrees that it may be time to say good-bye to all that.