I have built the Remington X23 boxcar. The kit as designed is virtually unbuildable - that is the thing would be so light and frail it would not stay on the rail, nor could you properly attach trucks to it.
I substituted .010" styrene sheet for the paper used, and built the car around a 1/16" basswood frame. So, it is virtually scratchbuilt. If you can scratchbuild, these provide a good start, although you'll need additional parts like brake gear, ladders, and grabs.
Here's a finished X23: