After Dark," which opened last night and will be repeated tonight at the Wolf Trap Barns, music director Richard Woitach sat at the piano in the bar adjoining the concert hall and kept the music going nonstop. There were differences between the intermission entertainment and the show itself, but they were fairly small. Woitach continued to play Weill, as he had done on stage as one of the show's four piano accompanists, and it worked splendidly in the cabaret setting.

Kurt Weill is probably the most notable crossover hit among classical composers of this century. A student of Ferrucio Busoni, with a violin concerto, two symphonies, chamber music and song cycles to his credit, he is also the composer of "The Three Penny Opera," "September Song," "Surabaja Johnny" and the Broadway hit of the 1940s, "Lady in the Dark."

The intermission's cabaret atmosphere was present frequently in the show, which was performed without curtains or scenery. The stage was bare except for two pianos, a few chairs and the 16 young, highly talented singers of the Wolf Trap Opera Company.

The show was divided about evenly between excerpts from "Mahagonny" (sung mostly in German) and from "Street Scene," sung in English. "Street Scene" had more of a pure Broadway flavor than "Mahagonny," which is material for an opera house, though somewhat ambivalent in its musical language. The artistic quality was equally impressive from both sources, and the "Street Scene" material often had a stronger impact because it was more accessible.

The material was neatly varied: at one moment, a group of New York tenement dwellers suffering colorfully from the summer's heat; a minute later, a pack of itinerant hookers searching for "the next whiskey bar" and "the next little dollar"; a quaintly old-fashioned pop song-and-dance in "Moon-Faced, Starry-Eyed," and a colorful, operatically elaborate scene dedicated to the glories of ice cream.

Fine performers were too numerous to discuss in detail, but Meredith Parsons, Dale Wendel, Jennifer Larmer, Lee Velta, Nicole Philibosian, Katherine Henjun and Colenton Freeman were among the names to remember.