"The difficulty here, it seems to me, is that there's precious little evidence to support the speculation that a steam-drill contest took place in Alabama - or anywhere else." - Lighter
There are two self-proclaimed eye-witness accounts, that of Neal Miller at Big Bend and that of C. C. Spencer at Dunnavant.
Miller's testimony is of very poor quality. He said the the contest was just a trial of the steam drill and that it was no big deal. He didn't watch it continuously, he just looked in on it from time to time as he went about his regular job. This is what he told Guy Johnson. What he told Louis Chappell is somewhat different. In his book, Chappell doesn't mention that Miller claimed to have witness the contest. In telling Johnson and Chappell about the contest, Miller gave different names and races for John Henry's shaker (turner).
Spencer's testimony is very detailed, but it has quality issues, too. Several of his dates are too early by a few years. He said that John Henry was from Holly Springs, Mississippi, and that the steam drill contest took place during work on the AGS RR. There is no appropriate tunnel on the AGS (Coosa (Spencer's "Cruzee") Tunnel is on the C & W) and I believe that Spencer misremembered "Holly Springs" for "Crystal Springs," Copiah County, where both Captain Frederick Yeamans Dabney and Henry Dabney, husband of Margaret, lived after the Civil War.
Spencer recalled a song about John Brown: "John Brown was a little boy / Sitting upon his Mother's knee / He said the Big Bend tunnel on the C. & O. Road / Will sure be the death of me." Evidently he did not connect this John Brown's death with a steel-driving contest with a steam drill. Sadly, we don't know the rest of Spencer's "John Brown" song. He declared, however, that John Brown, not John Henry, was the man of "'Big Bend tunnel fame.'" Most would probably think that Spence just got things confused, and maybe that's the case, but this makes me wonder about the possibility that "John Henry" is an adaptation of an earlier song connected with Big Bend.
In any event, Spencer got a lot of things right and partly right, including that "Dabner" was a boss on the job. Captain Frederick Yeamans Dabney was Chief Engineer for the C & W and in charge of its design and construction. Spencer is not the only one who recalled that John Henry and Captain Dabney were from Mississippi. In 1955 Warren Musgrove published part of an interview with Mrs. C. T. Davis, who claimed that his "boss man killed him in Mississippi after he left here," implying that the boss man (Captain Dabney) and John Henry went to Mississippi after the job at Dunnavant was finished. Thlis would be natural if Mississippi were home. (I doubt the truth of Davis' allegation, however. Certainly Crystal Springs newspapers don't report any killing involving Captain Dabney, whose family is mentioned frequently in the society columns.)
Unlike Miller's, Spencer's story is rich in now-documented detail. Although none of it concerns John Henry directly, the confirmation of Spencer's details lends versimilitude to his story. I consider his testimony much more likely to be true than Miller's.
No, I can't prove that John Henry and his contest are not fiction but I think the indications are contrary to that hypothesis.