Tyler struts his stuff on the amiable but derivative 'Rockin' on Top of the World,' a three-chord rave-up that finds the Aerosmith mouthpiece channeling a pack of howler monkeys, but it's Groban's syrupy, future American Idol favorite 'Believe' that serves as the soundtrack's commercial centerpiece. Composers Glen Ballard and Alan Silvestri provide the latter, relying on swelling choral arrangements, tender ballads, and polarizing yuletide offerings from Stephen Tyler and Josh Groban. The soundtrack to Robert Zemeckis' groundbreaking film adaptation of Chris Van Allsburg's popular children's book The Polar Express celebrates the season with both classic and contemporary holiday flair.