ATSF 3751 heads west on the BNSF transcon at Siberia, CA where the westbound and eastbound mains split for a few miles. ATSF3751 did not smoke very much on the run to the Grand Canyon but here she smoking a bit.
Good firemen will keep the particulate matter to a minimum for efficiency's sake and also to make the ride more pleasant for passengers. It's only the trackside railfans who like puffy plumes of particulate smoke. What you're seeing there is mostly unburned coal dust that causes boiler tubes to wear down prematurely, releases toxic carbon monoxide into the atmosphere, and rams cinders into the eyes of any passenger brave enough to open a window.