Quadruple track

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 or sections. Such conversion is referred to as "quadruplication".

A railway line with six parallel tracks, or a sextuple-track railway, has three tracks in each direction. The corresponding term is "sextuplication".

There are also instances of railway lines or sections with eight tracks, and cases with three or five tracks. All experience similar upsides and downsides.

Advantages

* Quadruple track can carry a larger amount of traffic, and through scheduling techniques like express trains can actually allow for more than twice the capacity of a double-track corridor. It is often seen around large metropolises or on busy inter-city corridors, with the most extensive urban usage seen on the New York City Subway.
* On quadruple track, faster trains can overtake slower ones, and so quadrupling allows all trains to reach their maximum speed. For example, high-speed rail with a 200 km/h average speed and commuter rail with a 60 km/h average can co-exist on quadruple tracks without interrupting each other.
* Maintenance and engineering work are easier on tracks in quadruple line with little resulting delay because standard double-track service can continue even if the other two double tracks are halted during the work.
* Quadruple track lines generally benefit from economies of scale in many other facets due to their larger capacities, from construction, operation, maintenance, storage, and services. Quad-track rail offers a cost-efficient model for high capacity rail service corridors.

Disadvantages

* Quadruple tracking costs more per mile, as it requires more materials and increased land acquisition costs. The required right-of-way is naturally wider, while interchanges and signaling become more complex, and track switching more frequent. This also applies to tunnelling and bridge costs.
* Maintenance costs are higher and often more complex, as there may be more switches on the track than on a two-track line, to allow moving between each of the 4 tracks.
* In order to maintain high speeds, grade separations are commonly required.

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 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 YorkWashington and PhiladelphiaPittsburgh 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. Future passenger and freight-separated track pairs are planned by the state of Virginia on the corridor between Washington, D.C. and Richmond, Virginia, and by California High-Speed Rail between Los Angeles and Fullerton, California.

* Other modes
Two separate double-track lines in proximity to each other, e.g., two double-track lines along opposite sides of a river, can operate as a quadruple track line. Examples of this can be found in Rhone in France and Rhine in Germany.

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 and fast in each pair) or by purpose (speed or direction in each pair). Pairing by direction allows the railway to interface to a double track more easily. With fast trains in centre, local stations can be on the outside, eliminating staircases for half the passengers. With slow trains in centre or when pairing by speed there can be a common platform for local trains with one staircase and one ticket booth.

Sometimes two of the tracks go more straight and with a little distance from the two other. This is a design decision when widening a double track section, and allows higher speed on the faster tracks.

File:Four Tracks Paired by Direction - Fast in Centre.jpg
File:Four Tracks Paired by Direction - Platforms in Centre.jpg
File:Four Tracks Paired by Direction - Slow in Centre.jpg
File:Four Tracks Paired by Use.jpg
File:Four Tracks Paired by Use SIS.jpg

Examples

Europe

Belgium

Several lines radiating from Brussels are quadrupled, for instance the Ghent-Ostend line as far as Essene-Lombeek. Further quadrupling has recently been carried out as part of the development of the Brussels Regional Express Network. The building of high-speed lines has also led to quadrupling - for instance the HSL 2 high-speed line between Brussels and Cologne runs inside the local lines as far as Leuven. Meanwhile since 1934 Brussels and Antwerp have been connected by two separate pairs of double track. Fast trains normally use line 25, while line 27 serves slow trains. In places they run parallel, but at times diverge and cross over each other.

Denmark

There are two places in Denmark with four tracks:
* Between Klampenborg and Høje Taastrup, through Copenhagen, Denmark, there are four tracks; two are for the separated S-trains and two for mainline trains; where the two tracks closest to Copenhagen (the oldest and central parts of Copenhagen) are reserved for S-trains (on much the lines this is the Northern and Western two tracks).
* Also between Høje Taastrup and Roskilde, where the two center tracks are for InterCity, long distance commuter trains (further than Roskilde or Ringsted), while the outer two tracks are for commuter trains to/from Ringsted or Holbæk