Powered Rails are used to either boost a Minecarts speed or to slow it down. When the Powered Rail has a Redstone Current going through it, Minecarts traveling over it will have the speed boosted, when no current is going through it Minecarts will be slowed down.

If one side of a Powered Rail is blocked and power is running through it, Minecarts placed on it will move at speed in the opposite direction of the Block.