I have never done this before, but I just found out how to make different loads appear on a car, such as putting a random shed on the TOFC car for no reason (which by the way, did work for me). I tried using a locomotive to see how that looked, and no matter what parts aren't realistic, I still liked the look of this:
I hit play to see the car without all those things showing up in the way. This shows up when I hit play.
I figure it has to do with the fact that it is not just a model, but a physical engine that should normally be able to be driven. Is there any way to get this to show up and not have any errors?