900 Miles and 500 miles APPEAR to be separate entities. Both Woody Guthrie and Cisco Houston recorded '900' and I believe Hedy West composed the tune 500 Miles. Her song was added to, and became a huge 'hit' in the sixties sung by Bobby Bare. Peter Paul and Mary (and virtually every other commercial folk group also recorded it)
900 Miles sticks pretty close to the Southern fiddle tune "Reuben's train" and is often played using a minor scale. The lyrics are very common and are found in dozens of other Traditional Mountain songs.
You can find the '900' melody a lot in the Bluegrass repertoire, as "Train 45".