Any good kits out there for detailing and lighting Athearn HO passenger cars?
My LHS sells interior detailing kits for streamlined Athearn passenger cars, but I am not sure about heavyweights. They are nice little kits. They come with a floor, partitions, and seats. Preiser seated passengers fit nicely without modification in the seats that come with these kits. If you want to contact them, it is Custom Railway Supply, 719-634-4616.
Edit: They could probably help you with lighting, too.
IHC makes these:
http://www.ihc-hobby.com/cgi-bin/bsc.cgi?sn=O765480688872U09294QW05F3O5129
They are very reasonable.
I just stuffed some of the IHC interiors into a bunch of ECW passenger cars. They’re meant to fit into IHC’s cars though…so I had to trim them down a bit. Only thing that grates is the colors–I really doubt railroads actually had car interiors that were pink and powder blue 
Do the Athearn cars still have the wheelsets with one uninsulated metal wheel and one plastic one? If so, you already have pickups - just ensure that all metal wheels on one truck are contacting the right rail and all wheels on the other are contacting the left. Now obtain a piece of wire and wrap it loosely around the axles (so that it makes a good contact but doesn’t stop them turning), then solder a fine wire to it and pass that through a hole in the floor. You now have a pair of power feeds for your lighting which can be done with either miniature bulbs or LEDs, though the latter may be trickier to wire. Good luck!
Check out Jay-Bee for lighting. One kit is made for Athearn cars and if you can put up with flickering lights they are ok. Although I’m not sure if they are still making the light kits as they have been out of stock at Walthers for many years now.
The IHC interiors are cheap and very nice. And after cutting and painting to fit the car you have they look great! I used IHC interiors for my Hi-Levels, and of course specific interiors are made for these now [xx(]. I even made a lounge interior…I just need the Hi-Level lounge [;)]
The interiors made for the Athearn cars are similar to the Hi Level interiors. I have constructed several of the Hi Level interiors from TSP, and they go together easy and look nice. It is a pain to go through and clean the seats up (flash). You may want to break down and go ahead and pick up a Overland Hi Level lounge. I have spoken with Mr Wright of Quality Wright Products, and Intermountain, and it looks as if the lounge will never happen. The Overland cars already have full interiors and lighting as well.