Check out Kasgro Rail <http://www.kasgro.com/specs.asp>, which specializes in building and owns a number of heavy duty, high capacity flat cars. The highest capacity cars they own are KRL 163200 and KRL 163201 [photos: <http://www.rrpicturearchives.net/rsList.aspx?id=KRL&cid=3&rid=163200>], which are 16 axle depressed center flat cars with a maximum capacity of 900,000 pounds (450 tons). As to the largest capacity car in the US, that would be a Schnabel car built for Westinghouse Nuclear to haul containment vessels, delivered last year <http://www.railwayage.com/index.php...sgro-builds-worlds-largest-railroad-car”.html>.