J.W. Foster in No More Silence, p212
“At the time the motorcade came through, there were about seven or eight people up there. As you looked down, I was standing over the third lane from the north curb of Elm Street. Four or five were standing right in front of me, and there were several on down the trestle away from me. Just prior to the shots, a three engine locomotive went by, so there wasn’t a lot that you could see or hear from up there even though the locomotive had already passed and just the boxcars were going by at the time the motorcade passed through.”
Foster mentions boxcars (my bolding) present when the motorcade came through. I wonder if some lower profile flatcars were in the train and possibly harder to detect in the McIntire photos.