I would not describe Casey Jones as wholly original. There is a long tradition of artists reworking American traditional music. Woody Guthrie described his own songwriting as a process where he would choose an existing melody and sing his words to that melody. Both Hunter and Garcia, had a deep love for and appreciation of American traditional folk music, (probably more so for Garcia). To describe the song as wholly original is to deny that aspect of Jerry Garcia's songwriting.

On the subject of the post, it is another difference between the Grateful Dead and Phish. Phish definitely did not draw their influences from Folk Music.