Just to clarify, thats not a tripleheader. Tripleheader would be three engines on the point. This is a Three-engine train, thus a doubleheader with likely a rear pusher ahead of the caboose. Looks like a stock extra, which was likely one of the few times they would run three engine trains cause they had to expedite the shipment, not wanting to make multiple hill turns.
My guess is that the train is close to maximum tonnage for these three locos on Cumbres pass, and that perhaps the crewman on the front has a an eye on the rails, and could be there to add some sand if they start slipping. Have never seen it done on the D&RGW, but its as good a guess as any as to why he is there. Neat shot either way!!
Greg