I had also come to the conclusion that the mystery photo was taken above the confluence of the Thompson and Fraser river. Along the Fraser river the vegetation is pretty much coastal and for most of the route the Canadian Pacific runs on the north side of the canyon
Finally the CP crosses the Fraser before Lytton and the confluence with the Thompson river
Along the Thompson the vegetation becomes much drier and the trees more sparse, and I suspect that the stone bridge is somewhere close to here
BTW, these photos were taken of 2816's first run east from Vancouver after it's re-build