I left Chama for Durango in May of 1981 after five years on the C&TS, where the use of the drifting throttle for braking as described in this thread was standard practice when bringing short trains (less than nine of the old box car coaches) down from Cumbres. When I started running on the D&S, I quickly learned another use for the drifting throttle--to pull the train. In the spring of 1981, the main throttle valves on both the 476 and the 473 were problematic at low steaming loads, as they wanted to be shut or wide open. When leaving Durango, once you got over the hump at 36th street, it was mostly near-level and tangent all the way to Trimble Lane. With the cantankerous throttles, running this part of the road was a real pain. However, the drifting throttle supplied just about the right amount of steam to keep the train moving at 15 to 18 mph. If the train began to slow down, a small bump with the main throttle would bring the speed right back up. Interestingly, the 478 had a smaller drifting throttle line (1-1/4" vs. 1-1/2" on the 473 and 476), which didn't allow quite enough steam to keep the train speed up. Fortunately, the 478 had a nice, smooth-operating throttle, so running at low loads with the main throttle was not a problem (so long as you didn't mind the wandering power reverse).
I also used the drifting throttle to pull the train through the High Line, westbound, which is slightly downgrade. I met with some criticism for pulling the train down grade, but I felt that by doing so I was keeping the slack stretched and giving a smoother ride. Whether that was true or not is open for debate.
In using the drifting throttle to pull the train, a concern could be raised about possible harm to the superheater tubes, as the firing rate is slightly elevated, and no steam is flowing through them to keep them from overheating. Evidently, the firing rate was not sufficient enough to put the superheater tubes at risk, as we never encountered any evidence of damage from overheating.
