Just received mine today. Interesting to note that both cars have separate Lionel catalog numbers as follows:
6-83155 - Smithsonian Flight Boxcar
6-83156 - Smithsonian Space Boxcar
But when packaged together they have the following Lionel catalog number:
6-83157 - Smithsonian Air & Space Boxcar 2-Pack
Another interesting note: If you search the Lionel (or any other place on the web), only the 2-pack shows up. Apparently Lionel is not going to break up the set - although I did find a couple web sites that had pictures of each boxcar separate from the set. The accompanying product info was for the 2-pack.