This is based on small torpedo cars used in the steel industry to cary molten metal. The one I have designed is a work in progress and still has some improvements to be made, but they are a ways off. For this Project, use 

The main thing I need to improve is the 3D printed trucks. If you'd like to use trucks that can actually couple to other trains, these ones by Atlas work great) https://mrmuffinstrains.com/products/atlas-66040-3rl-heavyweight-passenger-car-trucks-3-rail
You will also need two M3x12 screws to attach the truck to the BaseForTruck part. Some can be found here: https://a.co/d/c7vkWk9

If you'd like to use the 3D printed trucks, note that they don't have a coupler. Just insert 3 wheel sets (I used old Lionel ones and a bit of white lithium grease). 
These trucks are attached using the same BaseForTruck part, but instead use M4x12 screws to thread into the plastic of the ThreeWheelTruckAssem part. These can be found here: https://a.co/d/c7vkWk9.

Please reach out with any questions or feedback!