You guessed well; it is indeed at Fort Collins on June 10, 1959, on the 'private' trip Howard Fogg ran from Denver to Fort Collins and return for invited guests. The man in the white cap up on the tender is C&S road foreman of engines R.E. 'Mickey' Hansen, who generally ran the engine on every fantrip over the C&S. He can be seen in the cab of the photo I posted a couple of weeks ago taken on this same trip at Church's Lake and showing the engine's Chinese red cab roof.