James Jessie's shrinking machine.
Ironically large device used by James Jessie and his gang of British train enthusiasts to miniaturize stolen locomotives so they can run them on their model tracks.
Smart and 99 are inside one of the engines when the gang strikes and are shrunken along with it. However, they discover that ingesting yeast makes them expand, thus counteracting the ray's effects.
[Comic Strip Story #6 (Unofficial title: "The Not-So-Great Train Robbery").]