Doc and Merle Watson perhaps should be credited with floating the "longest train" verse into "Lonesome Road." It has no other connection with "The Longest Train."

"Lonesome Road" appeared in print in Scarborough, "On the Trail of Negro Folk Songs," 1925, collected from a Negro road worker in Virginia. Scarborough comments that she didn't know if the song was from black, or white, tradition. It doesn't have the train verse (p. 73, with short score).
Carl Sandburg collected a different version, perhaps from Texas. No train verse (The American Songbag, pp. 322-323, somewhat different music, and emphasis on a lying woman).
Collected by Brown in NC in 1923(?);, I don't have the lyrics or details. The music again varies.
Leach and Beck collected the song from African-Americans (1940s?) in Virginia; no railroad verses, but it has some other floaters (JAFL, vol. 63, p. 281, with score).