Centerbeam flatcars, centerbeams, center partition railcars or commonly referred to as lumber racks[1] are specialty cars designed for carrying bundled building supplies such as dimensional lumber, wallboard, and fence posts. They are essentially bulkhead flatcars that have been reinforced by a longitudinal I-beam, often in the form of a Vierendeel truss, sometimes reinforced by diagonal members, but originally in the form of stressed panels perforated by panel-lightening "opera windows", either oval- (seen here) or egg-shaped. They must be loaded symmetrically, with half of the payload on one side of the centerbeam and half on the other to avoid tipping over.