The swiftly-moving Ragtime: The Musical, now at the Cain Park Alma Theater, gives a musical snapshot of early 1900s in New York City. Based on E. L. Doctorow’s fine and dramatic novel with the same name, it features book by Terrence McNally, music by Stephen Flaherty and lyrics by Lynn Ahrens. The Cain Park production, directed by Joanna May Cullinan, pays appropriate homage to the classic America-as-melting-pot narrative (at least to the New York City, East Coast version).