According to the Traditional Ballad Index, the song was first mentioned in 1909 in JAFL, which would make it public domain, but of course many later recordings are copyright, including the 1924 release by Fiddlin' John Carson. Woody Guthrie had his own take on the lyrics, which would still be copyright; he did not compose the original.

Other titles include "Midnight Dew (The Train Runs a Wreck)," 1914 (music on p. 205 of Brown, vol. 5, North Carolina Folklore, which puts the tune (not Hoo chorus) in public domain.
I haven't checked through the 'Related Threads', above, but more info could be there.