This an articulated slag car that I built. I used 2 Walthers slag cars, an Athearn inter-mobile spine car coupling and 3 axle trucks.
This was a trial and error process. At first, I started with a 2 axle truck between the cars but it didn't fill the space to good.
I tried the 3 axle truck and it made the car look better, but I had to modify the cars so it would be able to pivot around curves.