About the 73′ Centerbeam Flat Car:

Centerbeam flatcars, centerbeams or center partition railcars 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. 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.