Minecraft wiki entry for Minecarts, associated text: A cart reverses direction when it collides with an object (wall, single block, player, other carts) while traveling on a powered rail. It does not reverse direction if it collides with a translucent block, such as stone slabs or glass. If a track including powered rails is bordered by blocks acting as "buffers", the cart continues back and forth along the track indefinitely. Having carts interact with each other on a short track designed this way can be used to chain multiple carts together as a "train". Once aligned, they all move together at relatively the same speed.