When a locomotive is spotted for sand at the sanding tower, in most cases it is also aligned to one of the coaling chutes. In the case of small power, like an 0-8-0 Switcher, coaling and sanding is a two-move process.
In the view above, a portion of the sand bin is modeled as a cut-away. There is a pit for “green” sand to be dumped into. The machinery house next to it provides power for the auger that transfers the wet sand into the pit and into the dryer (tall skinny structure to the left).
The machinery house also has the air compressor that supplies air to draw the dried sand from the dryer into the centre storage bin on the coaling tower. Another line runs to the header tank on the sanding tower to draw dried sand into it, from the coaling tower bin via underground pipe. The sand is then gravity-fed into the sand bin on the locomotive. The air lines are brass wire press-fit into holes in the structures. No glue is used on the piping so that the structures can be removed from the layout.
The sanding tower and sand bin are from Walthers, the dryer from Tichy, and the machinery house is included in the Walthers Concrete Coaling Tower kit.