It took me a while to figure out that the BT was for Barney Trucking, and that the trucks were making their appearance around the Convulsion Road interchange east of Salina, UT. Since it wasn't clear what they were trucking, I got on Google Maps to see if there was any evidence of mining in the area. I thought that maybe there would be open pit mining of some sort, but none showed up. However, there was a marker for the Sufco Mine. Turns out that the Sufco is a huge underground coal mine, and the trucks were carrying coal west to power plants. Here is the blurb from the Sufco web site:
The Sufco mine, which derives its name from its former owner, Southern Utah Fuel Co is one of the longest continuously running and most productive underground longwall mines in the U.S. It has been in continuous operation since 1941, initially as a room-and-pillar coal mine and since 1985 as a high-performance longwall operation.
The Sufco mining complex is located in Sevier County, UT approximately 30 miles north east of Salina, UT and 125 miles south of Salt Lake City, UT. It is located in the Blackhawk Formation in the Wasatch Plateau coalfield in which eight coal beds have been identified that contain coal seams more than seven feet thick.
Sufco has longwall and continuous miner reserves in the Upper Hiawatha and Lower Hiawatha seams in which it produces high-BTU, low-sulfur, compliance thermal coal.