Robert Zemeckis' computer-animated feature, based on the 1986 Caldecott Award-winning book by Chris Van Allsburg, is an extraordinary achievement, which isn't the same as being an extraordinary film. Van Allsburg's slim tale of a boy whose doubts about Santa, flying reindeer and the whole holly-jolly holiday ball of wax are banished by a trip on the magical Polar Express — a steam engine of such seductive grace and beauty that it seems more alive than the human characters — is long on atmosphere and short on plot. The screenplay, by Zemeckis and William Broyles Jr., overlays his delicate conceit with irrelevant songs and frantic but meaningless action. Alan Silvestri's musical contributions include "Hot Chocolate," sung by a bevy of hyperactive waiters, the insipid duet "When Christmas Comes to Town" and the generic "Rockin' On Top of the World," performed by a liver-lipped, animated Steve Tyler. And since there's never any question of the children being hurt, there's nothing at stake when they scramble atop the snowy tops of moving cars or the express is brought to a screeching halt by a herd of caribou. The adventure begins on a small-town Christmas Eve, where "Hero Boy" (voice of Daryl Sabara) — the children are all nameless — lies awake in bed, suspended between the desire to believe and the growing conviction holiday magic is an invention of well-meaning grown-ups. While straining fruitlessly to hear the tinkle of Santa's sleigh bells, he hears something equally amazing: the huffing of a train coming to a steamy halt in his front yard. Hero boy is invited by the genial but no-nonsense conductor (Tom Hanks, who was the motion-capture model for almost all the film's characters) to board and finds the train carrying several pajama-clad youngsters, including Hero Girl (Nona Gaye) and a self-centered Know-It-All (Eddie Deezen). It makes one last stop, on the wrong side of the tracks, to pick up Lonely Boy (modeled on Peter Scolari, voiced by Jimmy Bennett), a child scarred by a lifetime of Christmas disappointments. The Express navigates dark tunnels, roller-coaster climbs and drops, and a lake of dangerously thin ice before delivering the youngsters to the North Pole, where they're greeted by a benevolent Santa (Hanks). Clearly designed to be an instant holiday classic, the film's tone alternates awkwardly between slightly haunted, bittersweet nostalgia for childhood innocence and an aggressive, antic cheer that smacks of trying much too hard to convince everyone they're having fun. Similarly, the film's look vacillates between breathtaking beauty and creepiness. Snow, steam and the soft glow of lights in the darkness have rarely been so beautifully realized. But the film's characters, computer-animated over motion-capture footage of flesh-and-blood performers, are as blank-eyed and rubbery-looking as moving mannequins — the stuff of nightmares, not dreams.