Frictionless Cart

Cast at a minecart, it removes the friction from the cart for good. On plain rail it never slows down, it follows the track round corners, and it keeps going until you break it. An unpowered powered rail still brakes it, and a wall still stops it.

Higher levels raise its top speed: vanilla's own cap that never slows at I, five times that at II, ten times at III. Cast again at another level to change the speed.