I found this on the O gauge side and thought it might help out a bit in deciphering the new Lionel numbering system for our S scale trains.
The new numbers are typically seven digit numbers. Most now begin with "19". That number is to represent the year of production as in 2019. The number does not reflect the catalog year...but rather when the item will be produced. (That sounds optimistic).
The second two numbers are the marketing category.
So for our trains they are using 17-21.
The last 3 digits are are sequential numbers which represent the specific items offered in the referenced marketing category.
So using this American Flyer Legacy U33C from the 2018 Vol 2 catalog shows a stock # 1921011. So that number means 19 (2019 production)...21 (American Flyer Locomotives)...011 (ATSF #8500 U33C)
Notice that the (not shown) engine #8514 shows 1921012. That difference should distinguish that engine from the #8500.
Most folks won't care about the stock numbers new or old but being able to identify a specific item sometimes is useful.