Physics Girl: World’s Easiest DIY Electric Train.
These trains are so much fun! I’ve actually built one last year with students after I saw the instructions on a Swiss/German site that sells neodymium magnets:
Supermagnete.de: Der einfachste Elektrozug der Welt.
The hard part was finding copper wire without insulation, which doesn’t seem to be available in German hardware stores. If you try and build one, don’t be surprised if the battery runs out fast – it’s short-circuited by the coil, so the fun runs out after a short while.
If you need more inspiration, take a look at World’s Simplest Electric Train (2) by Amazing Science.