Johnny Cash's version starts off slow during the recitation.
Once he gets to the part where the train driver goes through the
gate, and starts "picking up speed"; Johnny then speeds up the song,
gradually until the tempo is really fast to the verse:
"Well I fooled you, I fooled you,
I got pig iron, I got pig iron,
I got all pig iron.
The Rock Island Line........"

Then it stays fast for the rest of the song.
Various recordings of Johnny's version ad-libs the recitations a bit;
and he normally kept it at a higher speed for the entire duration of
the song when performing live.