Cargo Wagon

It's just cargo...

This wagon is lovely and simple. Nothing more to it than passing electricity through connectors, and there is a thorough tutorial on how to do it. However, if you find it to be too much of a hastle for you, you can completely skip the electrical part. Only the coal wagon needs to transfer electricity to the locomotive. Any subsequent wagon doesn't neccessarily have to have electricity.

Wheels

It has eight wheels (two sets of wagon wheels) which rotate in their carriages 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 6mmx3mm neodymium magnets, also widely available on 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 :)