Ultimate geezerfest and rock-doc holy grail, Festival Express presents the long-lost footage of a forgotten Woodstock on wheels—the money-hemorrhaging summer-of-1970 bacchanal in which a pair of 22-year-old entrepreneurs contrived to place the Grateful Dead, the Band, Janis Joplin, and a half-dozen other acts on a chartered train across Canada.

The magical mystery tour stopped only for a series of riotous concerts—characterized by audience attempts to “liberate” the entertainment—or to replenish the booze supply. “Most of us were new to drinking at that time,” Dead guitarist Bob Weir notes in one of the split-screen survivor interviews that annotate the original footage. Given the heavy concentration of old folkies, the on-train jams are fascinating—particularly the zonked “No More Cane on the Brazos,” led by a howling Rick Danko and kept on track by Jerry Garcia’s smiling Buddha. Given the financial disaster that the festival would be, Buddy Guy’s cool, yelping version of “Money” is wonderfully appropriate. Nevertheless, one waits for Joplin, who, midway through the movie and two months from eternity, uncorks a performance of “Cry Baby” for the ages.

Starting with the scream on which a more conservative singer would climax, and then pushing herself to the far side of coherence, our Janis delivers an astonishingly wrenching and immediate performance. The most vivid evidence of her presence ever committed to film, it should re-ignite the age-old question: Was this doomed Port Arthur flower child the psychedelic Judy Garland or, pace Dusty Springfield, the greatest white soul-singer of all time?