When a cart is placed on a powered rail on an incline, the cart sits there and is not affected by gravity, whether the rail is powered or not.  (sometimes in survival mode when player-placed it goes down, never in creative or by dispenser).  This is a system that has been used to pick up entities at the bottom of the system.  I've attached an image, and there is a working setup in this video at the 17 minute mark, you need only watch for a minute or 2.  https://www.youtube.com/watch?v=iKxah_YU8JU