Passenger Wagon

Illuminated

... so your passengers wouldn't travel in the dark ...

Uses two small LEDs with a resistor. It is bidirectional so it can be attached with a connector on any side.

Electricity

Lights inside consist of two LEDs and one resistor soldered together, all of which are widely available. I have made detailed tutorials on the electronics part, so even if you hadn't had any experience with electronics you should be able to learn along and build this. Aside from providing light to passengers, this wagon also transfers electricity through its connectors to any other consecutive wagon.

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 :)