Perhaps someone from Lionel can tell us whether the true 6464 cars ( rivet pattern, metal door guides top/bottom, etc.) will ever be "available" again. As I said above, I've heard that Lionel will not do this mold again.
I'm not from Lionel, but I know the answers to your questions. Lionel box cars with full rivet detail, double metal door guides, etc. are available any time. They never went anywhere. The mold/tooling is readily available for use/production whenever anyone wants to make a car with that tooling. Feel free to share where you heard that incorrect information regarding the availability of that mold.