Quadruple track consists of four parallel tracks. On a quad-track line, faster trains can overtake slower ones. Quadruple track is mostly used when there are "local" trains that stop often (or slow freight trains), and also faster inter-city or high-speed "express" trains. It can also be used in commuter rail or rapid transit, usually with the two outer tracks carrying the local trains that stop at every station, while the inner tracks carry the faster express trains that stop at fewer, busier stations; this can also be reversed, with express trains on the outside and locals on the inside. At the local train stations, the express trains pass through the station at full speed.