of vehicles that are permanently or semi-permanently coupled together (such as the Pioneer Zephyr) is called a trainset. The term rolling stock is used to describe any kind of train vehicle. Components Bogies Bogies, also known in North America as trucks, support the wheels and axles of trains. Trucks range from just one axle to as many as four or more. Two-axle trucks are in the widest use worldwide, as they are better able to handle curves and support heavy loads than single axle trucks. Couplers Train vehicles are linked to one another by various systems of coupling. In much of Europe, India, and South America, trains primarily use buffers and chain couplers, while in the rest of the world knuckle couplers are used. Brakes Because trains are heavy, powerful brakes are needed to slow or stop trains, and because steel wheels on steel rails have relatively low friction, brakes must be distributed among as many wheels as possible. Early trains could only be stopped by manually applied hand brakes, requiring workers to ride on top of the cars and apply the brakes when the train went downhill. Hand brakes are still used to park cars and locomotives, but the predominant braking system for trains globally is air brakes, invented in 1869 by George Westinghouse. Air brakes are applied at once to the entire train using air hoses. Warning devices For safety and communication, trains are equipped with bells, horns, and lights. Steam locomotives typically use steam whistles rather than horns. Other types of lights may be installed on locomotives and cars, such as classification lights, Mars Lights, and ditch lights. Cabs Locomotives are in most cases equipped with cabs, also known as driving compartments, where a train driver controls the train's operation. They may also be installed on unpowered train cars known as cab or control cars, to allow for a train to operate with the locomotive at the rear. Operations Scheduling and dispatching To prevent collisions or other accidents, trains are often scheduled, and almost always are under the control of train dispatchers. Historically, trains operated based on timetables; most passenger trains continue to operate based on fixed schedules, though freight trains may instead run on an as-needed basis, or when enough freight cars are available to justify running a train. Maintenance Simple repairs may be done while a train is parked on the tracks, but more extensive