When Elvis Presley mixed black blues and R&B with white country in the mid-'50s, essentially bringing a new genre called rock & roll to the attention of the whole world in what was the pop equivalent of the Big Bang, few realized how long this little musical chemistry experiment had been fermenting. Presley had the looks, skill and charisma to make the kinetic hybrid of rock & roll reverberate in the public consciousness, but countless artists had been working this same territory since at least a decade earlier, as this extremely interesting collection makes clear. Songs like Billy Starr's startling and ramshackle version of "Hound Dog" falls wonderfully between Big Mama Thornton's original and Presley's sleek hit like the proverbial missing link, and there are countless other gems here, as well, like Hardrock Gunter and Roberta Lee's take on the Dominoes' "Sixty Minute Man," Zeb Turner's treatment of Peppermint Harris' "I Got Loaded," and Link Davis mowing through Roy Brown's "Good Rockin' Tonight" as "Have You Heard the News?" The joy and energy of new ground is obvious in every track, making Gravy Train more than a history lesson, and much more than a simple grafting together of stylistic genres. These hybrids rock, they swing, and they point the way to rock & roll in a manner that should have been as obvious as a train pulling into a station. A mystery train? Not if you had your ear to the ground.