Norm Cohen thinks that Travis added the Harlan and Hazard references. Archie Green suggests the same.

The reason is a little obscure, especially as Travis was not from anywhere near Harlan or Hazard--he was from Ebenezer, 200 miles away, which was a long distance in his youth. Archie Green speculates that he added the Harlan/Hazard reference because Harlan was a famous coal-mining town due to the violent disturbances there in the 1930s. So, even though he was from another coal-mining town, saying "it's a long way to Ebenezer," or even "it's a long way to Beech Creek" (where his dad worked) wouldn't have meant much to the public. But "it's a long way to Harlan" would immediately mean miners in most people's minds. Notably, it seems to be Travis who changed the song from a railroad song to a mining song.