Well this has been bugging me for a while as I found these boxcars in my caboose folder. They are the brown autoparts boxcars, one plain and one UP. So I started exploring the addons I have installed. All the free routes I installed except the P&O had the exe installer, which also comes with the complete folder structure of the addons. This makes no sense to me but I assume it is something from the Rail SIMULATOR days. It is like you get a double copy of everything. Anyway I installed the Tennessee Pass beta route and it is the culprit. In it's asset pak are the boxcars in the caboose folder. Further investigation states the route will crash if they are not there.
So to the scenario writers I suggest you do not use those two boxcars but use the real deafult ones. If someone doesnt have the Tenn. Pass installed they will not have them.