I am confused, Which "TMCC caboose" are we talking about? There are two versions:
1. Lionel 6-19889 (et all) Crewtalk cabooses.
2. Lionel 6-29834 and 35 Trainsounds cabooses. (If you don't believe me, look up the #.)
I personally own the crewtalk caboose. When I first got it, I attempted to "MU" it with a TMCC engine. It did not work. I left frustrated. I came back 5 minutes later and redid the "train" and it functioned properly. Just keep trying.
Sidenote 1. The crewtalk caboose works surprisingly well with RS5 engines. I thought they would be talking over each other, but they do not.
Sidenote 2. Anyone have a video of a trainsounds caboose they would be willing to share here or to me privately via email? I have been wanting one, but since I already have the crewtalk one, I am hesitant.
Sidenote 3. Yes! A legacy caboose would be a great idea.