Most people go by the MTH part number. According to that 2016 catalog,
20-97831 single car NORX coal porter
20-92079 six car set
20-92080 another six car set.
I did a search on eBay, Completed Items (so listed, may or may not have been sold), nothing. I think Completed is good for the last six months, so if someone sold it earlier than that, you would not know.
I did an internet search (I use DuckDuckGo) came up with this posting by MTH here on the forum
https://ogrforum.ogaugerr.com/...than-5-in-stock-list
saying the 79 and 80 had less than 5 in stock, implying the sets were made.
If you want to buy something, the Want To Buy section might be better, it should be under Miscellaneous
Good luck in your endeavors.