Train wheel rim measuring tool - GIF

[Toolmaker51]

Pure guess; if hollow run is depression of wheel profile by crown of track, qR might be remaining usable radius.
https://link.springer.com/article/10...864-017-0072-2
Warning; the page bar is like a 1/4" high, ultra long page of text.
I'll await Cliff Notes, For Dummies book, or the movie.....

Double-skin? Well, that's best left to the baby boy's parents.....