Coal Wagon

Purpose

Coal wagon is used to carry batteries. It supplies electrical energy to the locomotive and other wagons through magnetic connectors.

Battery cradle

The battery cradle is printed rather than relying on commercial prebuilt ones. You can probably find a commercially available one that can fit into the wagon if you don't want to go through the process of making the one from this set. In this set, cradle body is printed with a printer.

Wheels

It has four wheels (one set of wagon wheels) which rotate in a carriage to follow the shape of the track. These wheels are universal throughout all of the wagons.

Connectors

The two connectors are magnetic and they transfer electricity. Magnets are 6mm x 3mm neodymium magnets, also widely available in the market. Connectors are assembled with or without super glue. Once assembled and mounted connectors are extremely easy to use since they snap-on to one another. The direction of the two magnets in each connector will not allow accidental inverse polarity. If you do make a mistake of inverse polarity while assembling or mounting connectors, the voltage is very low and the components can handle inverted voltage.
DO NOT SHORT CIRCUIT THE CONNECTOR!

Think about the environment

I have done my best to design this wagon in such a way that it can be easily repaired and if really necessary salvaged. It is both sturdy but doesn't use too much plastic. So be responsible to the environment if things go wrong and try to reuse or repair whatever you can. Make it a challenge rather than nuance :)