i was the printer for a portrait photographer who was trained in the 20s/30s ..
and in the darkroom i used we had tray rockers. they were galvanized metal of some sort and ran on a slow motor. it had 3 platforms and they ran continuously all day long
( until i unplugged it ) it made quite a racket and was was a great set-up ..
i wish i could remember more details about it, other than it was a motor, and a cam that ran over 3 metal platforms. i miss all that automation !
unfortunately ( for me and her ) one of the rockers broke and stopped
so it was back to hand-work soon after.
i wish someone made something like this still !