The article by Garst summarizes his views, which also have appeared in Mudcat, in more detail.
Also worth reading for background is the chapter on John Henry in Norm Cohen, "Long Steel Rail," pp. 61-89, which includes a comprehensive discography of this much-recorded song.
Among the interesting items in Cohen is a copy of a printed song sheet, "John Henry, the Steel Driving Man," with the name W. T. Blankenship, c. 1900 (Cohen remarks that Blankenship seemed to be working from imperfectively remembered material).
Also included is a 1929 letter from Ernest V. Stoneman discussing John Henry. Stoneman believed that the song was set in West Virginia on the C & O, "the man that the song was made about was a negro who was a East Virginian." He goes on to describe the man, and mentions his song, "John Hardy," which he says was the man's real name. The West Virginia connection, discounted by Garst, was described fully in Cox, 1925, "Folk Songs of the South," chap. 335, 'John Hardy,' pp. 175-188 (Harvard Univ.; Dover unabridged edition).