Pete, my apologies; I misread your earlier post.
I checked the eBay product you linked, and it appears to be different from the one I purchased from LIONEL for use with the 15" UP passenger cars in 6-31712. This is a screenshot of the parts list for the "Chatham" coach included in that set listing the 2-pin connector.
The connectors look very similar and perhaps the 21" cars use a different connector. I have never had much luck chasing 21" passenger car parts on the website, not enough photos and too many "unavailable." I'll go back and check the 21" parts lists for the item and compare part nos.
Again, Pete, apologies for my misread.