A roller coaster train is a vehicle made up of two or more cars connected by specialized joints which transports passengers around a roller coaster's circuit. Roller coasters usually have various safety features, including specialized wheels and restraints. Description A roller coaster train is a vehicle made up of two or more cars connected by specialized joints which transports passengers around a roller coaster's circuit. It is called a train because the cars follow one another around the track, the same reason as for a railroad train. Individual cars vary in design and can carry from one to eight or more passengers each. Operation Many roller coasters operate more than one train, sometimes several, simultaneously. Typically they operate two trains at a time, with one train loading and unloading while the other train runs the course. On the Rock 'n' Roller Coaster at Walt Disney World, for example, there are five trains, but only four operate at a time (the trains are rotated out on a regular basis for safety reasons). Basic safety features Wheels Roller coaster trains have wheels that run on the sides (side friction or guide wheels) and underneath the track (upstop, underfriction, or underlocking wheels) as well as on top of it (road or running wheels); these lock the train to the tracks and prevent it from jumping the track. The side wheels can be mounted on the outside or inside of the train, depending on the manufacturer (although outside-mounted wheels are more common). The wheels are sometimes located between the cars, as well as at the front and rear of the entire train. Restraints Roller coaster trains also have restraints that keep the passengers in their seats. There are two major types of restraints: lap bar and over-the-shoulder. (Most modern over-the-shoulder restraints come into contact with riders at the lap; however, older so-called "horsecollar" ones actually restrain riders by the shoulders). Restraints always use two locking mechanisms, one on each side, for redundancy. If one fails, the restraint will remain locked. Most modern roller coasters also have seat belts that may act as secondary safety devices. On over-the-shoulder restraints, this seatbelt is mostly cosmetic as the restraint locks on its own. Lap bars were first used in 1907 with the Coney Island coaster Drop the Dip. Lap bar restraints consist of a padded bar mounted to the floor or side of the train that swings