Quadruple track

A quadruple track railway (also called four-track railway) is a railway line consisting of four parallel tracks, with two tracks used in each direction. Quadruple tracks 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 reduce congestion and some tracks are six in each direction.

Advantages

 * Quadruple tracks can manage large amount of traffic with usually twice the capacity of the double track. It is seen in large metropolis or big corridors.
 * In quadruple track, faster trains can overtake slower ones. It can contribute to the faster operation of the trains. High speed rail of 200km/h and commuter rail of 40km/h average can co-exist in quadruple track without interrupting each other.
 * It is relatively easy to do track maintenance and renewal work in quadruple line with minimal effect of train delay because double-track service is kept even if the two other tracks are affected for maintenance works.

Disadvantages

 * Quadruple track costs due to even requiring more materials and increased land acquisition costs. This also applies to tunnelling and bridge costs.
 * When adding tracks, land acquisition can become potentially expensive.
 * Maintenance costs are higher and often more complex as there may be more switches (points) on the track than the two-track line (to facilitate switching from outer to the inner lanes and vice versa)
 * For safety, grade separation tracks may be required.
 * If needing more capacity, it can be better to add a double track along a different route, because it could improve local and regional transit much along an under-served route, and reduce land acquisition cost by choosing a less built-up area.

Track operation
In quadruple track, trains are sorted in different ways in order to make maximum use of track capacity. These can include the combination of:

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. 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. When a quadruple track line divides to different destinations part way along, trains need to be sorted by their destination. Passenger trains and freight trains can be separated with each different track.
 * Sorting by speed
 * Sorting by distance
 * Sorting by destination
 * Sorting by passenger/ freight

A variation of this can be found on the quadruple track section of the Main Northern 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. Two double track lines along opposite sides of a river can operate as a quadruple track. Examples of this can be found in Rhone in France and Rhine in Germany.
 * Other modes

Ireland

 * In Ireland, the busiest section of the railway in the country, from Hazelhatch & Celbridge to Heuston was quadrupled in 2009.

United Kingdom

 * Significant lengths of West Coast Main Line, East Coast Main Line, Great Western Main Line, South Wales Main Line, South Western Main Line and Midland Main Line were even quadrupled due to high capacity and intercity lines.

United States

 * East coast
 * The New York Central's Water Level Route across upstate New York was four-tracked in majority as early as 1876, claiming to be the earliest 4-tracked steel main. This was extended to Buffalo by 1936. Financial troubles and changing traffic patterns caused this to be downgraded to a double track by 1975.
 * The Metro-North Railroad's New Haven Line is four-tracked from Grand Central Terminal in New York City to New Haven Union Station in New Haven, Connecticut.
 * Much of Amtrak's Northeast Corridor from Washington, D.C., to New Haven, Connecticut, is a four-track line, except for the segment between New Rochelle and the Sunnyside Yard in Queens, New York. In the Bronx, the Amtrak trains run separate from the New Haven Line. The right-of-way from Woodside, Queens, over the Hell Gate Bridge to Co-op City, Bronx, is three-tracked due to the presence of the single-tracked New York Connecting Railroad.
 * The Erie Lackawanna Railway had a four track main line on the former Erie Railroad, from its Pavonia Terminal in Jersey city to Suffern yard in Rockland County, New York. This allowed separation of freight and commuter trains. The EL Rwy also inherited former Lackawanna four-track Boonton Main Line from Dover, NJ, to Delawanna (Passaic), NJ.
 * The Central Railroad of New Jersey had a four track main from the CNJ Terminal in Jersey City to Phillipsburg, NJ, across from Easton, PA.
 * The Long Island Rail Road's Main Line is four-tracked from its East River Tunnels portal in Long Island City to Queens Village, NY.
 * The Pennsylvania Railroad had a four-track mainline carrying freight from Pittsburgh to Harrisburg via the Horseshoe Curve (Pennsylvania). This was how the name Broadway Limited came about from the "Broadway of a 4-track main." Much of the route between Pittsburgh and Paoli has been downgraded to three or two tracks.
 * Many lines of the New York City Subway are quadrupled – one of the few rapid transit systems in the United States to have extensive duplication. 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.
 * Reading Company's New York Branch between Neshaminy Falls and Yardley station was originally quadruple-tracked before being reduced to three tracks between Neshaminy Falls and Woodbourne station and two tracks between Woodbourne and Yardley. In 2016, the third track between Woodbourne and Yardley was relaid by SEPTA in order to separate SEPTA's West Trenton Line and CSX's Trenton Subdivision.


 * Midwest
 * The BNSF Racetrack in Chicago has a quadruple track section from Union Station to LaVergne.
 * The Chicago "L" has a four-track section on the North Side Main Line (Purple, Red, and Brown Lines) between Howard and Fullerton.
 * The Metra Electric District Line main line in Chicago is quadrupled between 11th street and 111th street.


 * West coast
 * Market Street in San Francisco had quadruple track streetcars for much of its length in the early 1900s – two tracks operated by United Railroads of San Francisco and two tracks operated by the San Francisco Municipal Railway (Muni). As the streetcar system was acquired by the government and was modernized to light rail, capacity was maintained by tunneling an additional right-of-way for Muni vehicles parallel, under the surface. The Market Street Subway's two subterranean levels are both double tracked, and the (retained) double tracked surface section runs heritage trolley cars for a total of six tracks in the same thoroughfare. The surface and upper level are laid at standard gauge and maintained by the San Francisco Municipal Transportation Agency while the lower level is Indian gauge track operated by Bay Area Rapid Transit.
 * Bay Area Rapid Transit is quadruple tracked for a short section at the approaches to and at MacArthur station in Oakland, California. The transit system had plans that called for Market Street Subway to be fully quadruple tracked to their unique operational specifications, with express trains running on a separate level than local trains; the final system was reconfigured due to budget constraints.
 * Caltrain commuter rail service was partially quad-tracked in short sections to enable Baby Bullet express service.

Hong Kong

 * The Tung Chung Line and Airport Express were quadrupled between Kowloon and Tsing Yi but share two tracks on the rest of their routes (until they diverge again before the western end). In addition, the West Rail Line runs parallel to Tung Chung Line and Airport Express but it has six tracks.

Timothy North

 * The Daniel and Jeremy Lines were quadrupled between Hainault and Fulton Street, but merged into one single track at Fulton Street, with the passing loop.

Australia

 * Even major railways in Australia also uses four tracks for shared - meaning with more trips.