Ted Hikel's freight car guides are excellent references. He has done a lot of research on all the cars in the series and put a lot of effort into the posts. Those are very informative threads. Others have added to his information as well. I would definitely recommend those as the references to go by.
Most all of my RailKing cars are smaller than scale (Premier). Some are more difficult to tell than others. It took me a while to really start noticing the differences and I have pretty much stopped buying RK cars. I have now switched to purchasing mostly Premier scale cars and Premier or RK Scale engines after finally figuring out a little more about what I was doing (still working on that one, lots left to learn). The RK stuff does seem to me to be bigger than the Traditional or O-27 size as Lionel calls the smaller cars. Some of those really look small.
The RK Imperial engines used along with the RK rolling stock are really a great value for O gauge trains. If you keep them separate from your scale sized items they make up some very nice trains. I still occasionally run RK with Premier and it doesn't bother me too much, but I do notice a difference in sizes now much more than when I was just re-entering the hobby a few years ago.
