The "two John Henrys" theory, which I believe is highly likely to be correct:

John Henry Martin was a highly reputed steel driver at Big Bend Tunnel, C & O RR, between Talcott and Hinton, WV, during its construction in 1870-72. He did not race a steam drill and his death was from "natural causes" many years later.

John Henry Dabney was a steel driver working on Coosa and Oak Tunnels, C & W RR, Dunnavant, AL in 1887. Steel-driving contests were popular recreational and betting events, and John Henry won all of those he entered. He raced a steam drill at Oak Tunnel, and won, but collapsed thereafter and died.

A ballad about JHD, which did not give his last name, was being sung in Georgia by 1888 and it was soon known in the Big Bend area. When it arrived there, people still remembered John Henry Martin and they began to associate him with the John Henry of the song, assigning to JH Martin the deeds of JH Dabney and localizing the ballad to "Big Bend Tunnel on the C & O Road."