A disadvantage of magnetic brakes is that they cannot completely stop a train, and as such cannot be used as block brakes. They also cannot be conventionally disengaged like other types of brakes. Instead, the fins or magnets must be retracted so that the fins no longer pass between the magnets. Accelerator Coasters, for example, have a series of magnetic brake fins located on the launch track. When the train is launched, the brakes are retracted to allow the train to reach its full speed. After the train is launched, the brake fins are raised to safely slow the train down in the event of a rollback. This is currently in place at the Top Thrill Dragster of Cedar Point and Kingda Ka of Six Flags Great Adventure, where magnets on the track lower during launch and raise to slow the train in such event.