Once you begin with "he picked up some of the lyrics from Negro railroad workers in Tennessee," it becomes hard to argue that he wrote the song. Unless you can show which parts were traditional and which were original, it sounds more or less like he recorded a traditional song, and then other people recorded the same traditional song. The extent to which the other people's recordings were influenced by Charlie's version can only be known if we know exactly what Charlie did to the song.

Did he ever tell anyone, or write down, what he remebered from the tradition? It would be interesting to know....