As noted by Gary Lucas over on his Flickennabok page, actually really well recorded!

Son House - Shetland Pony Blues

At 1:07 you can hear an actual steam-powered freight train chug past the recording studio in Klack’s Store Lake Cormorant Missippi where this classic Son House side was recorded by Alan Lomax. The makeshift studio abutted a railroad track. I played this for Don Van Vliet who remarked: “He’s singing about a Shetland pony and here comes the Iron Horse!” G.Lucas