Just a note here that most already know.
I got a MTH U30C from someone who had mounted the older regular KD couplers with metal boxes on it. Instead of cutting the back of the coupler box off, he cut off the 3 rail coupler's mounting tab on the front of the truck.
As it ran around the layout it had several issues like stalling and restarting, ignoring commands, etc. I put her on the bench looking for bad wires when I noticed that the wheels touch the coupler box as they turn. This created a dead electrical short. I got lucky as nothing was damaged.
I switched them to the plastic boxed KDs to be safe and used the shorter #743, I think they were? If you'd prefer to use the longer coupler for tighter radius and longer cars, I would stay with the plastic boxed ones. I haven't had one fail yet.
