I noticed that the "Black Girl" in the DT (which Spaw linked to above) is marked with the keyword "@infidelity." I'm not sure why, but I always imagined that the questioner in the first 2 lines is the girl's father. That doesn't seem to fit with the verse saying, "My father was a railroad man/Killed a mile and a half from town," but it could still be her mother. Anyway, I don't think any infidelity is necessarily implied. Fornication, maybe, but not adultery.

I found this information at The Deadlists Project:

According to "Southern Mountain Folksongs" by W. K. McNeil, there is an unpublished study of "In The Pines," running to 650 pages, by Judith McCulloh. This study analyses the lyrics and music of 160 variations of the song and identifies one musical and three textual elements that must be present for a song to be In The Pines. The textual elements are;
[1] the phrase "In the pines, in the pines, where the sun never shines"
[2] the phrase, "The longest train I ever saw"
[3] and one or more verses describing an accident in which someone is decapitated.

The following information comes from History of Rock Music @ The University of Tennessee:

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Looking Back Through the Pines...
Sound as history in rock music
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"Where Did You Sleep Last Night"
Nirvana
from Unplugged in New York
Recorded 1994

My girl, my girl, don't lie to me
Tell me, where did you sleep last night?
In the pines, in the pines, where the sun don't ever shine
I would shiver the whole night through.

Her husband was a hard-working man
Just about a mile from here
His head was found in the driver's wheel
But his body was never found.

My girl, my girl, where will you go?
I'm going where the cold wind blows
In the pines, in the pines, where the sun don't ever shine
I would shiver the whole night through.
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"Where Did You Sleep Last Night"
Lead Belly
Recorded 1944
[no text given]
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"In the Pines"
Bill Monroe and His Blue Grass Boys
Recorded 1952

The longest train I ever saw / Went down that Georgia line
The engine passed at 6 o'clock / The cab passed by at 9.

In the pines, in the pines, where the sun never shines
And we shiver when the cold wind blows.

I asked my captain for the time of day / He said he throwed his watch away
A long steel rail and a short crosstie / I'm on my way back home.

Little girl, little girl, what have I done / That makes you treat me so?
You caused me to weep, you caused me to moan / You caused me to leave my home.
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"Black Girl"
text as sung by Lizzie Abner
Collected by Cecil Sharp in Oneida, Kentucky (1917)

Black girl, black girl, don't lie to me
Where did you stay last night?
I stayed in the pines where the sun never shines
And shivered when the cold wind blows.
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Common Elements

Musically -- plaintive minor key, slowly-rocking triple meter, repetitious phrases
Text has three basic elements -- (1) chorus of "in the pines," (2) a stanza about "the longest train I ever saw," (3) a stanza about decapitation

Musical sound is also important -- (1) in country versions, the song's eerie qualities are emphasized by the "high lonesome" sound, (2) for Nirvana, Cobain's raspy voice echoes the dark emotions of the text as does the powerful "grungy" bass

Oddly enough, although "In the Pines" exists in many places on the Internet, I never found a version that seemed like a reliable transcription from Leadbelly. There are many more sites out there dedicated to documenting the versions done by Nirvana, the Grateful Dead, etc., than by Leadbelly himself.

I did find one site that said that the earliest recorded version was done by the Tenneva Ramblers in 1927, under the title "The Longest Train I Ever Saw."