The car shown up front in the picture is a flat car. It is the car behind the locomotive, barely showing at the right side of the picture, that appears to be a cattle car. In the movie, it comes chugging right across the center of the screen in close up, and you can get a good look at it.
I think the take away from all of this is is that it would appear to be incorrect to say that "there were NEVER any such things as open top cattle cars." Once can never say never.
And as Pallalin has indicated, one may be fairly certain that the burned out freight cars at the end of the Civil War were not promptly built right back to being freight cars. Cobbled together cars with just good side rails were easily built, very very useful, and probably in wide use for several years, particularly in the War devastated Confederate States (which included both Texas and Missouri).
Pallalin, yep you are right. Checked the movie again, blue not buff.