The K Line Chesapeake and Ohio Madison heavy weights that I have are all 18" (the length and car style are two things that I forgot to include in my original message), but I have a set of K Line (presumably of later manufacture) Chicago and Alton 18" Madison heavy weights that have full plastic interiors, ceiling lights, and clear plastic windows. A couple of years ago at the York Meet I bought a Lionel 18" Chicago and Alton Madison coach that was probably manufactured within the last five years. It has the full interior and the Lionel car body appears to be about 1/8" longer that the K Line. I have heard rumors, but never really confirmed, that the newer Lionel 18" Madison cars with full interiors/ceiling lights, and clear plastic windows were made from the old K Line molds. But given the length of the Lionel car and the fact that it will not fit into the K Line packing styrofoam, I am not sure this is true.
I am replacing the diaphragms in the Chesapeake and Ohio K Line heavyweight cars as the originals have pretty much disintegrated and are beyond my repair capabilities.
Once the interiors/diaphragm questions concerning the K Line C&O heavyweights are settled, I plan to install ceiling lights similar to the lighting in the Chicago and Alton K line 18" cars or possibly the Lionel Chicago and Alton car. This will be a new area of expertise that I will address at a later date.
I primarily model the railroads that operated in Central Illinois from 1930-1950. The C&O cars are for a young relative who has recently become interested in O scale 3-rail and wanted the K line passenger cars. I am trying to further this interest into a hobby - thus, the attempts to convert the passenger cars.
Thanks for the assistance.