Great photos - thank-you.
BTW, these are by Myron Davis for Life Magazine in May 1943 - it was clearly a unit move to a training area, I believe somewhere in the Southwest. Over 30 photos of this move are in the Life Archives, at least a few after unloading. SP rolling stock dominates, but there are two PRR Gons, at least one MILW box car, and some other cars where the heralds can not be made out.
The unit had a few small artillery pieces - if I recall they are a smaller caliber that was quickly deemed ineffective for use against the axis - by 1943 I believe a large caliber gun was being shipped to units overseas... what they trained with was a good gun for training - why waste the latest model for US training when better weapons were desperately needed in North Africa (at that time in the war.)
I suspect one way to distinguish unit training moves versus export moves is the presence of guards on the train. Once a unit signed for equipment, they made sure no-one took any of it (there are pictures of the train underway in the Life archives - every flat car had a soldier assigned to it - often riding in an open jeep - some with the windscreen up, others with the windscreen down....)
Conversely, security of new equipment for export was probably the responsibility of the RR, and monitored by the crews and RR police.