In Elizabeth Cotten's "Willie" and in many versions of "Frankie and Johnny," the conveyance taken to the graveyard is a "Rubber tire buggy" and "rubber-tire hack." (I have never heard them referred to separately, interstingly.) Mississippi John Hurt refers to the two (in "Hot Time in the Old Town Tonight"), but not in reference to burial. On the other hand he is going with a razor to square accounts with "that man" so perhaps he's referring to death indirectly, along the lines of "he's going to need a rubber-tired buggy and a rubber-tired hack when I'm done with him." But that's conjecture.

At any rate, there's the background. My question: Why are rubber-tire conveyances especially appropriate for going to bury people? Is a iron-bound wheel just not respectful enough for the reposed?