Since we are talking about Passenger car parts, I just wanted to chime in here. Most of the things people said the need are already available. You can get ends---doors and roofs are in the works. all parts will work with USP products. The roofs are being made and will be available in about 90 days. It is my understanding they will be a one piece casting done by an outside vendor and sold by a different person.
Most people including myself, agree the core kit USP sells for O scale will not work because of scale it will buckle. This is why the roof is in the works.
these are O scale parts.
Hope this helps
Bob Harris