They're a way to transport people from one location to another. The train itself is a big carriage on large wheels, with engine to pull it along. Then there's tracks on the ground that the train drives on, so that it follows the same path every time. It makes it safer to move large loads and many people at once.

Now, most trains have the usual precautions like thick window covers that bolt shut and the usual cat patrols, but where they get really interesting are the ones that have sawblades on them.