Multi-car train for payload?

I've gotten map payload to the point where it uses the well GP35, and it can be pushed all the way down the map in about 10 minutes as scout. So, for added fun, I took the flatcar model and tried to parent it, or to ballsocket it to the GP35. But TF2 won't load the map, it just quits out on me, without getting to the map. Before the flatcar, things were fine.... What's my problem?

Oh, that makes sense. Okay, straight out, I'm a railfan. The engine that attacks you in Well is the GP35. It's basically a mid-size diesel locomotive.