Where does “boxcars” come from?

I appreciate the detailed instructions, but I notice a significant issue: the etymology chain provided traces the word "six," but the headword is "boxcars." These don't match. Boxcars (a dice term for rolling two sixes) would derive from "box" + "cars," where "cars" is slang for the dots on dice. This etymology chain for "six" is relevant to understanding *why* two sixes are called boxcars, but it's not the direct etymological chain for the compound word "boxcars" itself. Could you clarify: 1. Should I write about "six" using this chain, or 2. Should I write about "boxcars" (and if so, do you have the etymology chain for that compound)? I want to provide an accurate summary sentence rather than one that misrepresents the etymology.

boxcars (English): plural of boxcar; API pl; Double six in a dice...