How To Play Freight Train by Elizabeth Cotten
As far as I know this is the original recording, and the recording I've based my transcription on:
And here's a great performance of the song when she was in her later years. Notice her left-handed, upside down style - bass is all played with the index finger, melody with the thumb.
I've written out the music and tab below for the two versions of the song I demonstrate in the video. Firstly there's the basic arrangement. Then there's a more or less note-for-note version of what Cotten plays on the first section of her recording above, though it's impossible to exactly replicate what she does when playing the guitar the conventional way.