The MTL HOn3 C&S coal cars seem to have a different draft gear box than the MTL HOn3 reefers mentioned in the Conder/Niemeyer/Zook article, so I thought I'd share my experiences with them.
The coal cars have draft gear boxes which are perhaps closer to the MTL N-scale boxes used on the Trout Creek log cars: the Sharons are a very loose fit on the posts, and the boxes (such as they are as they have no sides at the front where the couplers emerge) are attached to the cars with plastic pins.
I thought about sleeving the posts as described in the HOn3 Annual article, but the draft gear boxes aren't remotely prototypical anyway, so I decided to toss them and go with the Accumate boxes.
The screws supplied self-tap nicely into the plastic pin holes (which are also in the right position), but the screws are too long. I tapped the holes with the screws whole, and then cut their points off. (Once the holes are tapped, the screws thread in fine without their points.)
The Accumate draft gear boxes also need shortening right in front of their back holes.
Other than that, it's all pretty straight-forward.