"The Little Black Train" mentioned 4 songs:
  - Hell Broke Loose In Georgia - p. 129
  - "The Little Black Train" - pp. 131, 134, 140
  - Many Thousands Gone - p. 134
  - Sourwood Mountain - p. 135

The first and last two songs are, like many of these songs have been, mentioned only briefly and in passing as songs John performed. Hell Broke Loose In Georgia is a famous "old-time" song popularized by the Skillet Lickers. You can read more about the song here, though I will comment that the Fresno State and Mudcat's research on the tune is rather lackluster. You can listen to the Skillet Lickers' version here. Many Thousands Gone (aka No More Auction Block or just Auction Block) is an interesting choice of song to mention. Almost every version of the song I've found is very blatantly anti-slavery, making the song's inclusion virtually the only overtly political comment in the Silver John series. Perhaps Wellman included it as a commentary on the mountain regions' opposition to the Civil War and slavery in general? The song is not exactly less political than other slave-related songs collected in the area, most notably Those Cruel Slavery Days, as reported by Fields Ward (no professional recording online, but the New Ballard's Branch Bogtrotters verion, Heritage HRC-CD-116 - 1995, is a very authentic one). You can read more about Many Thousands Gone here and here on Mudcat, or here. Popular modern versions of the song were recorded by folkies Bob Dylan and Pete Seeger. Sourwood Mountain is a popular fiddle and banjo tune often found in the Appalachian Mountains; you can read more about it here or here, and listen to the first commercial recording by Uncle Am Stuart here.

The Little Black Train

"I heard a voice of warning,
A message from on high,
'Go put your house in order
For thou shalt surely die."

"Tell all your friends a long farewell
And get your business right--
The little black train is rolling in
To call for you tonight.' "

"A bold young man kept mocking,
Cared not for the warning word,
When the wild and lonely whistle
Of the little black train he heard."

" 'Have mercy, Lord, forgive me!
I'm cut down in my sin!
O death, will you not spare me?'
But the little black train rolled in."

"Go tell that laughing lady
All filled with worldly pride,
The little black train is coming,
Get ready to take a ride,"

"With a little black coach and engine
And a little black baggage car,
The words and deeds she has said and done
Must roll to the judgment bar."

"Oh, see her standing helpless,
Oh, hear her shedding tears.
She's counting these last moments
As once she counted years."

"She'd turn from proud and wicked ways
She'd Leave her sin, O Lord!
If the little black train would just back up
And not take her aboard."

This song, about a black train which haunted the property surrounding Ms. Donie Carawan's land, was the manifestation of tormented spirits created after Donie cheated on her fiancé, High Fork Railroad owner Trevis Jones, with a man named Cobb Richardson, who worked on the HFR. People believed that Donie convinced Cobb to kill Trevis, who had already willed her his railroad operation and a large plot of land. Cobb was caught, but refused to implicate Conie in the murder. After Cobb's execution, his mother, Mrs. Amanda Richardson, spoke the curse into existence, and the train continued to haunt Conie until she (quite literally) faced her demons.

Though one recording of the song made by Joe Bethancourt employed his own rhythm, believing the song to be a Wellman original, it turned out to actually be a traditional song. You can read more about the song at Fresno State here or on Mudcat here or here (featuring Gargoyle's prescient comment that "links are BS in the long run"). You can hear a nice recording by the Carter Family here.