thats an interesting image, as far as i know, there was no railroad at bonanza so, if its a Uintah building it either has to be dragon, watson or mack. I had a go adjusting the exposure on the cars , no markings on them, they might be Uintah cars as they do look quite low from the photo i have of the building at mack, which has standard gauge cars next to it , I'd say that these are lower than standard cars loading into the ng freight houses.
s for which building it is, the mack freight houses had numerous narrow doors, whereas the dragon and mack examples had fewer. the distance between the doors makes me think its one of these and not mack.