“In Transit,” the last new Broadway musical of the fall season, is supposed to be set underground in the New York subway system. But the place is so sanitized, so sparkly clean that the equally scrubbed characters sit on the stairs to sing without saying “Yech.”

The show, which advertises itself as Broadway’s first a cappella musical, begins by telling us “Every sound in this show is made by the actual human voice.” The next moment, we realize that the sounds made by a hand mic held almost inside an actual human mouth also will qualify.

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The 100-minute musical — really, a collection of songs about mostly unrelated people in career and romantic transition — is meant to be inspired by the rhythms of the subway. This comes through best with the rumbling beats and virtuosic sound effects made by Boxman, a wise subway eccentric and one-man band played by beatbox artist Steven “HeaveN” Cantor.

Otherwise, it’s a stretch to find the subway-rhythm idea as more than a pretext for getting together 11 actors to portray 40 very sincere and familiar characters — the struggling actress/office temp (Margo Seibert); the obsessed dumped girlfriend (Erin Mackey); the engaged gay guys (Justin Guarini and Telly Leung), one of whom is in the closet to his pious but kindly Texas mother (Moya Angela); and the former Wall Street guy (James Snyder) fired for pressing “send all” on an indiscreet email and now, unbelievably, left without enough to take the subway.

More important, they are there to explore tricky harmonies in more than a dozen simple songs with simple structures about “being stuck,” “moving on” and, finally, “what it’s like to be here instead of getting there.”

The show, efficiently directed by triple Tony winner Kathleen Marshall, has book, music and lyrics by four friends who met after college in the increasingly mainstream a cappella scene. One is Kristen Anderson-Lopez, half of the married team that wrote the songs for “Frozen.” According to the program, the piece has been based on “an original concept” by six people — those four plus two others.

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And the arrangements have been made by Deke Sharon, vocal producer of the “Pitch Perfect” movies and TV’s “The Sing-Off.” The credentials of the show are impeccable. I just wish it were a fraction as challenging theatrically as a morning commute.