These cars are built-to-order by shapeways.com (based on 3-D designs supplied by Eric Cox of Panamint Models).
Here's what Eric has to say about them -
This Baltimore & Ohio 45-foot coach was built in 1856 to the design of Henry Tyson, the railroad's master of machinery. The model is based on an often-reproduced contemporary drawing to which three common-style contemporary roof ventilators were added. The correct trucks and link & pin couplers are included. All that's needed to finish is wheelsets (.540" axles, MT or similar) and .010" wire for the endrails, for which locator holes are provided. The ends are also made to accomodate Republic Locomotive Works etched brass end platforms with railings. Trucks mount with #0 screws. A Microscale Z/Nn3 coupler can be substituted for the link and pins, using the same mounting holes.