I said I'd post pics when I cleaned up the inserts. Well, here you go!
So, you can now drop in some coal into your carts. They fit perfectly and remove easily. One more thing I can tick off my todo list!
What's remaining on my todo list? I need to tweak the wheels to look a little more 3d - increasing contrast on the wheels might do. I need to try to increase the size of the tabs on the tapered mine cart. Right now, they're so fiddly that they were not fun to assemble. I'm ok with something being hard to assemble, but these were unnecessarily so. And, then I need to create some step-by-step instructions, and I need to invent some branding.