The mainline BART network operates six types of electrically-operated, self-propelled railcars, built from four separate orders. The first four types, built from 1968 until 1996, total 669 cars (although 662 are currently available for revenue service), and have two sets of passenger doors on each side of the car. The newer two types, which are technologically incompatible with the older types, are in the process of manufacturing, delivery, and commissioning, and are due to replace all older types by 2022 while simultaneously expanding the fleet for future extensions. They will all feature three sets of passenger doors on each side of the car to speed up passenger boarding.