According to Greenberg's Prewar set book, the dark gray 318 was released in a passenger set with 319, 319, 322 maroon passenger cars. Set 342(a). In the 1924 chapter it says they didn't have a boxed example and the catalog was released before the cars were available as that is the first year for the 319, 322 cars. The catalog shows orange cars, probably O gauge 710/712.
That is the only mention that I see of the dark gray 318. PS: I'd love to have one! Congrats.