The trash trains mentioned are basically run as unit trains. BNSF's starts in Everett, WA, makes a pickup in Seattle, then runs thru to Roosevelt, WA., where a train of empties waits for the return trip. Ewe - Pee's train starts in Seattle, makes a pickup in Tacoma, WA., ( and Centralia I believe ), then heads straight to their unload near Boardman, OR. Pretty efficient operations by both RR's.
Also consider that the Boise area is only 500 miles from Seattle or Tacoma, WA. This is only a day's drive via truck. I know of several truckload container carriers who haul into Walmart's distribution center in Hermiston, OR., then deadhead or find a break - even load to Boise for a return backhaul to Puget Sound. Don't know if Ewe - Pee could compete with trucks on running time, except in severe winter weather. Just a few thoughts...