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A quadruple track railway (also known as a four-track railway) is a railway line consisting of four parallel tracks, with two tracks used in each direction. Quadruple track railways can handle large amounts of traffic, and so are used on very busy routes.
Some tracks are only tripled, having only one extra track to relieve congestion, while some tracks are sextupled, i.e., six parallel tracks with three tracks in each direction.
Advantages of quadruple track
- Quadruple track can manage a larger amount of traffic with usually twice the capacity of double track. It is often seen around large metropolis or on busy inter-city corridors.
- In quadruple track, faster trains can overtake slower ones, and quadrupling can contribute to faster operation of trains. High-speed rail of 200 km/h average speed and commuter rail of 40 km/h average can co-exist in quadruple track without interrupting each other.
- It is relatively easy to do maintenance and engineering work of tracks in quadruple line with minimum effect of train delay because double-track service is kept even if the other two double tracks are halted during the work.
Disadvantages of quadruple track
- Quadruple track costs more due to requiring more materials and increased land acquisition costs. This also applies to tunneling and bridge costs.
- When adding tracks, land acquisition can become prohibitively expensive.
- Maintenance costs are higher and often more complex as there may be more switches (points) on the track than on a two-track line (to facilitate switching from outer to inner tracks and vice versa).
- For safety, grade separations are almost always required.
Quadruple track operation
In quadruple track, trains are sorted in various ways in order to make maximum use of track capacity. These can include one or a combination of:
- Sorting by speed
A faster express line and a stopping local line are separated, with each having a separate pair of tracks. Construction of new double tracks dedicated to high-speed rail alongside existing conventional double track used by regional and local passenger trains and freight trains is a form of quadruple track. It increases the capacity of that route significantly, and allows for significant increases in inter-city high-speed train frequency with reduced travel times.
- Sorting by distance
Long distance inter-city rail and freight trains are separated from short distance commuter rail. This helps to prevent delays on one service affecting the other, and is commonly seen in metropolitan areas. Quadrupling may be necessary when a new commuter rail service begins to operate on an existing line. Sometimes the local trains have separate technology, such as electrical system or signalling, which requires strict separation, for example in Berlin or Copenhagen.
- Sorting by destination
When a quadruple track line divides to different destinations part way along, trains need to be sorted by their destination.
- Sorting by passenger/ freight
Passenger trains and freight trains can be separated with each different track.
A variation of this can be found on the quadruple track section of the Main North railway line in New South Wales between Waratah and Maitland where one pair of tracks are used exclusively for coal trains and the other pair are used for passenger trains and general freight. A similar process, but with all intercity and commuter passenger trains on the outer tracks and thru-freight trains on the inner tracks, was done by the Pennsylvania Railroad on its New York-Washington and Philadelphia-Pittsburgh mainlines prior to the takeover of operations by Amtrak and Conrail (and later Norfolk Southern). This is somewhat still done to this day by NS, CSX, and Conrail Shared Assets trains on Amtrak-owned trackage in the Philadelphia area.
- Other modes
Quadruple track layouts
As it can be seen from the pictures below in the Gallery of diagrams, the four tracks can be paired either by direction (slow/fast in each pair) or by purpose (speed or direction in each pair).
- The New York Central Railroad's Water Level Route was the first long distance 4-track railroad in the world.
- Much of Amtrak's Northeast Corridor from Washington to New Haven is a four track line.
- The Pennsylvania Railroad had a four-track mainline carrying freight from Pittsburgh to Harrisburg via the Horseshoe Curve. This was how the name Broadway Limited came about due the "Broadway of a 4 track main."
- The BNSF Racetrack in Chicago has a quadruple track section from Union Station to Lavergne.
- The Chicago L has a 4-track section on the Purple, Red and Brown Lines between Howard and Fullerton.
- New York City Subway – many lines of the New York City Subway are quadrupled. Hence, many express services are operated in the New York City Subway. Express trains and local trains are separated with each different track.
- SEPTA's Center City Commuter Connection in Philadelphia is quadruple track, as is much of Philadelphia's Broad Street Subway.
- Significant lengths of the West Coast, Great Western and East Coast Main Lines in Great Britain are quadruple track, with the remainder of the lines being double track. These lines are high capacity and high speed lines running from Glasgow, Bristol and Edinburgh to London.
- The London Waterloo to Southampton / Salisbury main line South Western Main Line from London Waterloo to Worting Junction, Great Britain.
- In Ireland, the busiest section of railway in the country, on the approach to Dublin's Hueston station was quadrupled in 2009.
- The Berlin Stadtbahn, Germany, has four tracks. Two are for the separated S-Bahn and two for mainline trains.
- Between Klampenborg and Roskilde, through Copenhagen, Denmark, there are four tracks. Two are for the separated S-trains and two for mainline trains.
- The İstanbul–Ankara Main Line has a quadruple track section between Etimesgut and Kızılay in Ankara, Turkey.
- Most of the railway through Stockholm County, Sweden has four tracks, sometimes having two routes. Still the central section has two tracks only, with two more under construction (Stockholm City Line).
- The Amsterdam–Arnhem railway between Amsterdam Bijlmer and Utrecht Centraal, the Netherlands, has been quadrupled between 1999 and 2008.
- The Breda–Eindhoven railway between Boxtel and Eindhoven, the Netherlands, was quadrupled between 1998 and 2002.
- The Rome–Naples high-speed railway and the Rome–Sulmona–Pescara railway in Italy combine to form a quadruple track section between Roma Prenestina and Salone.
- The main section of Ferrovie Nord Milano line between Milan and Saronno in Italy. Outer regional trains are segregated from the inner suburban trains.
- The 112 km long Hamm–Minden railway between Hamm and Minden in Germany is completely quadruple-track with separate tracks for freight and passenger trains.
- The Mumbai Suburban Railway has quadruple track from Chhatrapati Shivaji Terminus to Kalyan railway station on central line and from Churchgate railway station to Virar railway station on western line.
- The Tung Chung Line and the Airport Express in Hong Kong are quadruplicated between Kowloon and Tsing Yi stations, but share two tracks on the rest of their routes (until they diverge before the western end). The two lines shared two tracks when they were opened in 1998.
- The Ocean Park Cable Car system got two pairs of ropeways.
- The Guangzhou–Shenzhen Railway is quadruplicated for its entire length separating passenger with freight traffic. It is the first railway in China to do so.
- The Shanghai–Nanjing Intercity Railway runs parallel to the slower conventional Nanjing-Shanghai Railway for most of its length. Other times it parallels the Beijing–Shanghai High-Speed Railway, effectively making it a 4-track corridor a majority of its length.
- The Gyeongbu Line in South Korea is quadruplicated on 84.9km on its route, and sextuplicated on a further 11.7km
- Hankyu Railway in Osaka has a sextuplicated section between Umeda and Juso stations (2.4 km).
- Keihan Main Line in Osaka is quadruplicated between Temmabashi and Neyagawa Signal Box (~13 km). 
- Seibu Ikebukuro Line in Tokyo is quadruplicated between Nerima to Nerima-Takanodai stations (3.5 km).
- Between Tokyo and Odawara (JR East) 83.9 km is paired by use (not including Shinkansen).
- Tokyo – Shinagawa 6.8 km: 6 tracks
- Shinagawa – Tsurumi 14.9 km: 4 tracks
- Tsurumi – Yokohama 7.1 km: 6 tracks
- Yokohama – Totsuka 12.1 km: 4 tracks
- Totsuka – Ofuna 5.6 km: 6 tracks
- Ofuna – Odawara 37.4 km: 4 tracks
- Between Tokyo and Omiya (JR East) is paired by use (not including Shinkansen)
- Tokyo – Akihabara: 4 tracks
- Akihabara – Ueno: 5 tracks
- (Tokyo – Ueno 3.6 km)
- Ueno – Nippori 2.2 km: 10 tracks
- Nippori – Tabata: 4 tracks
- Nippori – Oku: 4 tracks
- Tabata – Akabane: 4 tracks
- Oku – Akabane: 2 tracks
- (Nippori – Akabane 7.4 km)
- Akabane – Omiya 17.1 km: 6 tracks
- Between Kusatsu and Nishi-Akashi (JR West) 120.9 km (not including Shinkansen)
- Kusatsu – Kyoto 22.2km is paired by direction: 4 tracks
- Kyoto – Umekoji – Mukomachi 6.4 km is paired by direction: 5 tracks
- Mukomachi – Ibaraki 21.8km is paired by direction: 4 tracks
- Ibaraki – Suita is paired by use: 6 tracks
- Suita – Shin-Osaka is paired by use: 8 tracks
- Shin-Osaka – Osaka – Tsukamoto is paired by direction: 4 tracks
- (Ibaraki – Osaka 14.6 km)
- Shin-Osaka – Miyahara – Tsukamoto: 2 tracks
- Tsukamoto – Hyogo is paired by direction: 4 tracks
- Hyogo – Takatori is paired by direction: 5 tracks
- (Osaka – Takatori 38.2 km)
- Takatori – Nishi-Akashi 17.7 km is paired by use: 4 tracks
- Between Ochanomizu and Mitaka (JR East) 21.5 km is paired by use.
- Ochanomizu – Yoyogi: 4 tracks
- Yoyogi – Shinjuku: 8 tracks
- Shinjuku – Mitaka: 4 tracks
- Between Kinshicho and Chiba (JR East) 34.4 km is paired by use.
- Kinshicho – Nishi-Chiba: 4 tracks
- Nishi-Chiba – Chiba: 6 tracks
- Between Ayase and Toride (JR East) 29.7 km: 4 tracks/paired by use
- Between Osaki and Komagome (JR East) about 20 km is paired by use.
- Osaki – Yoyogi: 4 tracks
- Yoyogi – Shinjuku: 8 tracks
- Shinjuku – Komagome: 4 tracks
- Between Souen and Heiwa (JR Hokkaido) about 9 km
- Souen – Sapporo is paired by use: 3 tracks
- Sapporo – Heiwa is paired by direction: 4 tracks
- Between Niigata and Kami-Nuttari (JR East) 1.9 km: 4 tracks/paired by direction
- Between Imamiya and Tennoji (JR West) 2.2 km: 4 tracks/paired by direction
- Between Inazawa and Nagoya (JR Central) 11.1 km/paired by use: 4 tracks
- Between Hiroshima and Kaitaichi (JR West) 6.4 km: 4 tracks/paired by direction
- Between Orio and Moji (JR Kyushu) 24.6 km
- Orio – Kokura: 4 tracks/paired by use
- Kokura – Higashi-Kokura 1.6 km/paired by direction: 6 tracks
- Higashi-Kokura – Moji is paired by direction: 4 tracks
- Japan Railfan Magazine (Koyusha) No. 478 p. 36
- Japan Railfan Magazine (Koyusha) No. 478 p. 37
- Japan Railfan Magazine (Koyusha) No. 478 p. 10
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- Japan Railfan Magazine (Koyusha) No. 478 p. 29