A flatcar is a railway wagon with a floor but no roof and no high sides, used for goods. Trains use it for things that are too long, too heavy, or too oddly shaped for a box: timber, steel, machines, even other vehicles. You chain the load to the floor so it cannot slide. It is American railway English; in Britain people more often say flat wagon. It is a work word from freight trains, not passenger talk. Though actually the “car” is not a car you drive — on American railways, car just means a wagon.