If you are wanting to build a freight train, I like Athearn’s freight cars. If passenger is your desire, Bachmann sells some really nice and inexpensive cars.
some useful links
http://www.walthers.com (get their catalog, it is the modeler’s bible.)
http://www.caboosehobbies.com (Best online and walkin hobby shop anywhere!)
see my train blog at http://wwwjoe-daddy.blogspot.com/