Minecraft wiki entry for Minecarts, associated text: A rail that is "off" slows carts passing over it as if by friction. A single inactive rail is enough to completely halt a cart in most cases. Carts that are loaded and/or at full speed cannot be stopped by just one block, but two "off" rails can stop it. On a long downslope, a stretch of three inactive rails are required to be sure of halting a speeding, loaded cart.