From the recording Follow the Drinkin' Gourd

Although this song is fairly well-known and a wonderful one to sing, its provenance is unclear. Very little is known about whether there was a "map song" used by slaves to escape, and it's more likely a 20th century creation.  When I learned the song I was unfamiliar with the "Peg Leg Pete" aspect which is often included, and so I've left it out.  But regardless of whether it really dates from ante-bellum days or whether its story is accurate, it's a great song, and a wonderful image.  Many, many slaves attempted escape and many succeeded, but most were from the border states such as Maryland or Missouri. The mention in the second verse of "...when the sun comes back and the first rail calls..." refers to when the hours of sunlight increase, and birds begin to return.  Traveling at this time was safer, perhaps because ice and snow were gone.  A rail is a shorebird which is very slender.  'Thin as a rail' refers to this, rather than to a fence post.