The rolling stock of the Bay Area Rapid Transit (BART) system's heavy rail component consists of 662 electric multiple units, built from three separate orders. To run a typical peak morning commute, BART requires 579 cars. Of those, 535 are scheduled to be in active service; the others are used to build up four spare trains (essential for maintaining on-time service). At any one time, the remaining 90 cars are in for repair, maintenance, or some type of planned modification work. All trains on the separate automated guideway transit line are in regular use without spares. Bombardier Transportation is manufacturing a complete replacement of the fleet, due to be delivered in batches by Fall 2021. There will be 775 vehicles in total, with long term goals of eventually increasing this to 1,081