The Classic Railroad Hand-car is designed to be a fun way to give children a sense of engineering intrigue as they propel themselves along the tracks (sold seperately). They can easily see how linear motion is turned into rotational, and then ultimately to their movement along tracks. It helps kids stay active, and teaches about environmentally friendly ways to get around. This toy is meant for children ages 12 and up.
This toy's main issue is safety. Somehow the working components need to be visible, but at the same time we don't want kids getting their hands stuck in it. Plexiglass might be our friend for this one. It would also have to be speed-limited, as we don't want kids flying off at any sort of serious velocity. With such safety precautions, this toy would most likely cost a lot to make.