One of my pet hypotheses has bit the dust.
Letitia Dabney, Fred's youngest sister, left a memoir in which mentions the family's loyal slave, "Henry." According to Letitia, Henry was born ca 1844.
The dead hypothesis is that this Henry took the surname "Dabney" after the Civil War, moved to Copiah County, where Captain Fred Dabney lived by 1880, and became the legendary steel-driving man.
I now realize that Letitia's "Henry" is probably the "Henry Page," 26, who was in the Augustine Dabney household in Raymond, Mississippi, in the 1870 census. Unless he changed his name or Spencer erred, he cannot be the "John Henry Dabner" described by Charles C. Spencer as the steel-driving man.
Thus, I cannot argue that Captain Dabney became close to John Henry as John Henry grew up in the Augustine Dabney household in Raymond.
This still leaves the Henry Dabney/Dabner (1870, 1880 censuses) of Copiah County as a candidate for the steel-driving man. This man was probably one of Thomas Dabney's 150-odd slaves at Burleigh Plantation. Fred Dabney was often there, so he probably knew many of Thomas' slaves. Indeed, in 1870 Fred is enumerated as living at or near Burleigh.
Thus, it is still possible that Fred Dabney knew John Henry Dabney in Copiah County, Mississippi, well before the boring of Coosa and Oak Tunnels for the C & W (C of G) at Dunnavant, Alabama, in 1887-88.