As you say probably one stop at the bottom of the steep grade in the Sumpter Valley at S Wye water tank where the retainers could be turned down, the engines watered and wheels cooled. Max grade descending below this point is 1.5 and in the Bowen Valley grade is as little as .72 dropping to .36 at the south end. They might have made another water stop at Boulder Gorge, but possibly not for a down grade move. We can take it as fact that they did run a very large train at least once in this manner as I don't doubt the main part of the story and they did it all with hand and whistle signals!