Magnetic brakes are made up of one or two rows of neodymium magnets. When a metal fin (typically copper or a copper/aluminum alloy) passes between the rows of magnets, eddy currents are generated in the fin, which creates a magnetic force opposing the fin's motion. The resultant braking force is directly proportional to the speed at which the fin is moving through the brake element. This very property, however, is also one of magnetic braking's disadvantages in that the eddy force itself can never completely hold a train in ideal condition. It is then often necessary to hold the train in place with an additional set of fin brakes or "kicker wheels" which are simple rubber tires that make contact with the train and effectively park it.