I don't know for sure, but I think Intermountain may have done the tooling for Red Caboose. I think that Intermountain did, or had done their own tooling.
As far as I know Red Caboose did the cars mentioned above. I did not know that their stuff went into Atlas O. I did know that Intermountain O scale went into Atlas O.
One note if you buy these kits, the truck width and required axle length is shorter on Red Caboose cars than Intermountain. Intermountain uses the same length axle as Athearn and Weaver. The metal Intermountain wheelsets drop right into their cars. for Red Caboose if you want to replace the wheelsets you have get something from NWSL that fits.
These kits are not difficult to build but they are tedious. As mentoned, there are a lot of fragile small parts. It takes a little patience and time to get everything installed on the body. They are not "shake the box" but they are not craftsman style either.