As was often the case ,Woody played his 900 miles with a minor scale over major chords. Listen to his "Pastures of Plenty", or "House of the Rising Sun". Leadbelly also did this on a number of his songs like "Out on the Western Plains". It's a disconcerting sound when you first hear it, but once you get used to the "modal" feel, it's bloody marvelous. One of the best examples of how this approach affects a song is in Flatt and Scruggs' "Foggy Mt. Breakdown". The original Mercury recording had Flatt playing an E major chord (when asked about this, he bristled at the suggestion that he didn't know an E minor chord!) and the tune was electric! In later versions they used E minor and to my ears, it was far less exciting.