Atlas had two types of HO-scale wheelset standards: short and long. The diagram is from about 15 years ago, and the company used different standards (“for caboose” is just a name).
Here is a discussion from 12 years ago.
Plastic to metal wheelsets - Model Railroader / General Discussion (Model Railroader) - Trains.com Forums
P.S. I made that statement with my memory still hazy. I’m sorry.