The advantage of cardboard over other materials is that it's easy to upcycle. Cutting and folding the cardboard material is not hard, you can cut it to pretty any imaginable shape with regular scissors and, it can also be arranged easily. Summer is coming, and your kids will have plenty of times so you should find them some excellent do-it-yourself projects. We have discovered fifteen ideas of trains made with recycled cardboard that they will love, from small ones to big ones; we are pretty sure that one of those trains will catch your eyes! :)
This miniature cardboard train will give a lot of fun to kids.
This locomotive was made using plain cardboard but need real skill to be made.
This beautiful steam engine can be done for some KG school project and then placed inside the house or in a kid room. And, it is so huge that kids can get inside it.
If you have collected a large quantity of cardboard, you can make a significant cardboard upcycled train like this one. (source)
This locomotive is made with upcycled cardboard, then old CD’s were used for the wheels, and the engine was wrapped with old newspapers.
If your kids prefer a subway than a train, this is a beautifully done cardboard subway decorated with photographies to mimic a real subway.
You can even add some wheels to your train; two old trailers were used to make is moveable. (source)