NOW THAT WE'RE spending several billion dollars to build Metro, how shall we use it? A subway has possiblities , after all, that lie well beyond the daily trudgery of commuting and shopping. Consider, for example, what you can do at lunchtime with a subway and a sunny day. If you get off at Gallery Place, the escalator will bring you to a street corner with the Martin Luther King Library on one side and the National Portrait Gallery on the corner. If you walk around the gallery, you'll further find that a block of F Street has been closed and rebuilt with benches, trees and, currently, a crop of daffodils. It's a fine place to eat a sandwich.

Then, wiping the mustard from your fingers, you might take a look at the inside of the gallery. The portraits of the Presidents are pretty bad, but no need to waste your valuable lunch hour on them; the other things are interesting, and the gallery's placards offer an unusually helpful commentary.There's an exhibit of the Winold Reiss portraits that illustrated Alain Locke's 1925 anthology, "The new Negro," one of the first assessments of the Harlem Renaissance. The placard tells you that in 1907 - not, in general, a good year for equal opportunity - Locke won a Rhodes Scholarship and went to Oxford, before taking his doctorate at Harvard and coming here to teach at Howard for 40 years. That's worth knowing.

Continuing down the same corridor - impelled by curosity and, conceivably, a disinclination to go back to the office - you find Mark Twain exhibit. Nothing startlingly new, but there's a fragment of correspondence between the author and his friend William Dean Howells, who Twain considered a final authority on taste. The question concerned a line in the draft manuscript of "Tom Sawyer" in which Huckleberry Finn, complaining about life at the widow's, says that the servants harass him with compulsory decencies "and they comb me all to hell"

Twain told Howells that he had read the passage to his wife, who didn't object. Neither did her aunt or her mother, "both sensitive and loyal subjects of the Kingdom of Heaven,so to speak." But what did Howells think? The answer came back by return mail: "I'd have that swearing out in an instant . . . It won't do for the children." That was in 1876.

Incidently, if the benches on F Street get crowded, you might try the gallery's interior quadrangle, where chairs and tables are set out on the grass.

To be cerebral about it for a moment, a couple of interesting things are happening here. Although it wasn't part of Metro's original cost-benefit analysis, the subway is serving the arts and letters by putting the gallery and the library within lunchtime reach of anybody in walking distance of any subway stop. Conversely, the increased traffic and attention can only be good for the level of the exhibits. All that, for only 80 cents round trip.