I had bought the MTH 30-2392-1 Redbird 4-car Subway Set and the two car add-on set. I kept the 4-car set but during a lean year or two I sold the add-on set as well as many uncataloged MTH Boxcars.
When I look on the the auction sites, I rarely see them, and when I do, the price is often 2 to 3 times what I paid.