Supertramp's hidden gems are a product of ‘70s pop at its most theatrical: The staccato electric piano and overlapping voices of “Child of Vision,” for example, could come from the West End stage. Many of their songs feel like narrative journeys, whether it's the sparkling folk-pop of “Sister Moonshine,” the anguished soul of “I'm Beggin' You,” or the showstopping redemption ballad “Downstream.”