The building next to the sand box is where the sand is dried by a stove or heater system. When a locomotive needs sand, the loco provides the compressed air to blow the sand up the pipes from the heater building into the sand tower where it is then gravity fed to the locomotive sand bunkers.
I built what is similar to an ash pit on a track that is also my team track where covered hopper loads of sand are dumped into the pit and moved to the holding box via an auger conveyor. My sand loader is atop my drying house.
Early on I believe it was shovelled by hand out of low sided gons.
I've seen pics of conveyors alongside the tracks in later photos.
As far as getting it into the dryer house maybe an auger or again shovel?