Based on other negatives in the group, this is an eastbound stock extra. Because of the line's position on the south side and the relatively deep canyon, this has to be between Bridge 320 (aka Bridge B) and Sapinero.

An eastbound stock extra somewhere in the canyon between the Crystal Creek bridge at mp 328 and Bridge 320 just past Curecanti.

Another view of the same unknown stock train making its way through the Gunnison Canyon.