The sanders on these engines are directly fed from an air valve in the cab. The air lines run up to the sand dome and to a separate sander nozzle for each sand pipe. Occasionally something will clog and we have to work a little to get them cleaned out and working again. This is why you sometimes see little piles of sand where the engine has been standing. There is no way to operate each sander individually so while we are working on one the rest are blowing sand onto the rail as well.
Then there is also the possibility that someone turns them on and forgets. Not that this would ever occur.