of the block. Like trim brakes, block brakes can also be used to control the speed of the train. These are usually known as mid-course brake runs. An example of a mid-course brake run is on Titan at Six Flags Over Texas, which consists of such a brake run (which slows the train down drastically, almost to the point of stopping) due to the severe G-forces in its downward 540-degree helix following the brake run. Mid-course brake runs give more time for another train to be loaded. Types of brakes Various types of braking exist when dealing with roller coasters, some of which have been recently developed due to technological advancements in design. Skid brakes Skid brakes essentially involve a long piece of material, often ceramic-covered, situated in the middle of the track parallel to the rails. When the brake is engaged, the skid raises and friction against the underside of the train causes the train to slow and eventually stop. Skid brakes were one of the first advancements in roller coaster braking and are typically not utilized in modern creations with the exception of Twister at Knoebels Amusement Resort in Elysburg, PA and the Matterhorn at Disneyland in California. Fin brakes Fin brakes involve a metal fin being attached to the underside of a train. The track is fitted with two computer-controlled squeezing mechanisms which upon closing, squeeze the fin and either slow or stop the train. Fin brakes are the most common form of brakes on roller coasters today. Sometimes they are thick metal box beams (mostly on Bolliger & Mabillard roller coasters); others are thin metal plates. They slide between pairs of friction pads similar to automotive brake pads. Fin brakes are designed to be fail-safe, so that a loss of power will cause them to engage. Brakes are constructed according to a certain measure of redundancy, meaning the ride is usually fitted with one extra set of brakes to bring the train to a hold even if one brake fails. Opening is done by a bellows type of air-operated actuator. Each set of brakes is fitted with its own air supply system controlled by supply valves which open the brake when it is safe to do so. A heavy spring, usually made of steel, is used to hold the brake closed by default. Magnetic brakes Rather than slowing a train via friction (such as fin or