Thanks for this information! This is all very interesting considering how little research has been done into Utah & Northern rolling stock.
Another possibility for the source of the oddball underframe without westinghouse brakes might by the Utah Eastern, which ran from Salt Lake City to Park City. It had two way cars constructed in the Utah Central Railroad shops in Salt Lake City in 1882, Way Car No.2 and Way Car No.3. Both were "borrowed" in 1885 by the Utah & Northern when Union Pacific forcibly took over the Utah Eastern. I've been trying to track down information on them for the PacificNG site but very little appears.
Utah Eastern Rolling Stock
Utah Central Car Shops