A brake run on a roller coaster is any section of track meant to slow or stop a roller coaster train. Brake runs may be located anywhere along the circuit of a coaster and may be designed to bring the train to a complete halt or to simply adjust the train's speed. Contrary to some belief, the vast majority of roller coasters do not have any form of braking on the train itself, but rather forms of braking that exist on track sections. One notable exception is the Scenic Railway roller coaster, which relies on an operator to manually control the speed of the train. On most roller coasters, the brakes are controlled by a computer system. Some older coasters have manually operated friction or skid brakes, some with a pneumatic assist. These are either engaged at the control panel or operated by pulling or pushing large levers in the station. Types of runs Trim brakes Trim brakes are sections of brakes which are intended to adjust a train's speed during its course rather than bring the train to a complete stop. They may be engineered into a ride at its design stages at certain anticipated troubled spots, or later retrofitted once it is discovered that trains traverse certain areas at higher-than-anticipated speeds. Trim brakes are often either added for safety reasons, to lower g-forces in certain areas, or for maintenance/mechanical reasons, to lower the cost of wear-and-tear damage (especially on wooden roller coasters) caused by the trains traveling at faster than normal speeds. Usually, a proximity sensor precedes the trim brake in order to identify the current speed of the passing train. From this, the trim will then grab the train's brake fins to reduce the train's speed to that set by the control system. Block brakes Block brakes are sections of brakes located on any roller coaster wherever more than one train is intended to run. They act as virtual barriers between the trains running on the roller coaster, preventing collisions should one train stop along the course for any reason. Block brakes must be capable of completely stopping the train (should a vehicle preceding the block stop) and starting a train (after it has been stopped). Block brake sections usually start the train again either by using a slight downward slope to let gravity take its course, or by using drive tires to push the train out