Diesel–hydraulic locomotives use one or more torque converters, in combination with fixed ratio gears. Drive shafts and gears form the final drive to convey the power from the torque converters to the wheels, and to effect reverse. The difference between hydraulic and mechanical systems is where the speed and torque is adjusted. In the mechanical transmission system that has multiple ratios such as in a gear box, if there is a hydraulic section, it is only to allow the engine to run when the train is too slow or stopped. In the hydraulic system, hydraulics are the primary system for adapting engine speed and torque to the trains situation, with gear selection for only limited use, such as reverse gear.