Unzip these into your Nationals folder:
Cardinal Auto Parts: A small track inside an auto junkyard. Really easy.
http://www.mcmfactory.com/display.php?cat=tracks&id=3170
Suburbia: A large, winding, supercross-style track in a housing addition.
http://www.mcmfactory.com/display.php?cat=tracks&id=3171 (http://www.mcmfactory.com/display.php?cat=tracks&id=3171)