It was a particular feature of the soda fountain that just came to me. On the back wall, behind the aisle that the waitresses walked up and down taking and delivering orders, and in front of the usual equipment of coffee urns and malt mixers and dispensers for drinks and such, there was a trough a few inches deep with a conveyor belt in it, running back to a small room where the dishes were washed. The dirty dishes would be placed on this conveyor and be whisked away to be cleaned. The trough was deep enough that they could not be seen, except that the straws in the drinking glasses would stick up above the rim. I remember sitting at the counter when I was very young and watching those straws go by. I never tired of it.
Strange. I have no idea what put that image into my mind.