Once the cart is placed on a rail, the rail will explode. Random entities will be created; potions, bats, mobs with pumpkins on their head, and rarely Withers.
It can only be spawned in through creative mode.
(Please edit the following text because there is probably more) so far it has been seen in: The Ultimate Modpack
Note: this page has not been completed, so it is most likely lacking information and might be hard to read.It also harvest crops if fully grown.