One of the guys I know around me is modeling St Gen & the Mike & Ike.
I was all over the MP, mostly around Houston, Van Buren and N Little Rock, AR, but traveled all over (El, Paso, Brownsville, Ft Worth, Kansas, etc.) A couple regional/district and system level jobs. Actually didn't get over much of the M&I, just the DeSoto Sub and around Poplar Bluff.
Not very clear picture of a MP car, due to the shadow. Note the bottoms of the hoppers don't have a "tube" and are flat. They have a flat gate with am "angle" on each side that is the slide for the hopper gate. They will also have a smaller tube that has a coupling on the end so crank the gate open. Its basically a rod with a gear on it that uses a rack and pinon to crank the hopper gate open and closed.
Better pic of a model, similar type car, right type gates:
The big tube in the center screams plastic pellets or powdered chemicals that are vacuumed out of the car.