Tunesmith;
It's true that Jack did play a similar style of picking to Libba Cotten but "right side up". However I think that most of us got it from Peggy Seeger who was here in the late 50's - as was Jack - when we attended her guitar and banjo classes. That is what most of us aspired to. Freight Train picking was referred to by us newcomers to the style as Clawhammer because that is the term Peggy used.
If you refer to The Penguin Book of American Folksongs compiled by Alan Lomax Page 150, he describes Clawhammer style of guitar playing.
I don't have the time or inclination to test it out right now but it ain't what we learnt under that description. I must admit That I have never heard anyone use the phrase clawhammer guitar for years and years.

In my experience of hearing and visiting and watching various banjo pickers the terms clawhammer, frailing, framming, knockdown and probably more all seem to be interchangeable. Plus very few people play in exactly the same way.

GSS. Don't get your bowels in an uproar over what people call the style. It seems quite common currently to mis-name various styles of music. Take for example The Blues, I see postings on here from time to time about great new blues on you tube. I take a look and find something entirely different or just a humble attempt to copy.
Inexcusable really now that virtually everything ia available on re-issue CD's.

Sorry if I rambled on a bit

Hoot