Because of the unique bolster thickness on the cars and trucks that Lionel uses, it takes some effort to replace their trucks with "scale" trucks. I have found that Atlas O trucks worked on a couple of cars with some shimming for the coupers.
I just replace the wheelsets with Intermountain or Athearn wheelsets and reuse the lionel trucks. It does involve disassembling the trucks and cutting off the tab for the large lionel coupler. I have done well over a dozen cars this way. The older trucks with the stamped bolsters are better in my opinion because y ou can bend the bolsters slightly to tighten up the space between journals if the wheels are too loose.
Doing this the wheels tned to be a little sloppy, but I have had no derailments or problems with this.
