Where has "On the Twentieth Century" been for the last 37 years? If, indeed, the 1978 musical-comedy has been saving itself for just the right mad screwball alchemy, then, all right, this was worth the wait.

Ego, love and the lovingly egomaniacal theatricality of theater people are the subjects of the Roundabout Theatre Company's breathlessly charming revival of a show about which, I admit, I never before have felt the slightest bit bonkers.

But the real subject of director Scott Ellis' production is expertise. From the virtuosic, wildly endearing Peter Gallagher as fading Broadway director Oscar Jaffe and the equally dazzling Kristin Chenoweth as movie queen Lily Garland through every supporting cast member, the show builds respect -- showstopper by showstopper -- for Cy Coleman's terrifically demanding operetta-and-jazz-inspired music and for some of the most clever lyrics ever written by that cleverness machine, Betty Comden and Adolph Green.

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Or maybe the songs and the story just seem first-rate because Ellis and choreographer Warren Carlyle treat them that way, lavishing detail upon comic detail on characters who, face it, have been around since Ben Hecht and Charles MacArthur turned Bruce Millholland's story into a hit play in 1932.

The time is still 1932 and theater folk are traveling from Chicago to -- just maybe -- Broadway on a swanky deco train (designed with nonstop creativity and cartoon glamour by David Rockwell) with passengers and porters in luxurious costumes (by William Ivey Long).

Gallagher, returning to the show after a virus, looks a little pale. But his baritone booms impeccably as he infuses the failing Svengali with the joyous, diabolical glint of desperation. Chenoweth, in a role created for Madeline Kahn, picks up the torch and dashes off with it -- glittery coloratura and selective vibrato blazing -- with all the unpredictable comic timing of a dainty trucker.

Mary Louise Wilson is deliciously dry as the religious fanatic who may fund Oscar's show. Mark Linn-Baker and Michael McGrath, as Oscar's agent and press agent, make these Tweedle-dums of Shubert Alley seem as if nobody ever dreamed them up before. And Andy Karl, star of last season's "Rocky," has fun with his pumped-up muscles as Lily's ambitious young lover, an actor.

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Ellis actually has him pump iron with the tiny actress as a barbell. And, over and over, Carlyle turns the four train porters into a marvelous tap-happy quartet that recreates the sound -- and the almost preposterous pleasure -- of the long-lost cross-country carriage trade. Lovely, all lovely.

WHERE American Airlines Theatre, 227 W. 42nd St.

INFO $67-$147; 212-719-1300; roundabouttheatre.org