It appears the 'senior' member of this EMD duo is providing most of the pull as an SP manifest heads west through Woods Cross on UP's Salt Lake Subdivision. The 'youthful' SP 8559 is an SD40T-2, built new by EMD for the SP in Feb. 1979 while the 'senior' SP 3841 is a GP9E, built new by EMD as SP GP9 No. 3706 in April 1959.