Adouble-track railway usually involves running one track in each direction, compared to asingle-track railway where trains in both directions share the same track.
In the earliest days of railways in the United Kingdom, most lines were built as double-track because of the difficulty of co-ordinating operations before the invention of thetelegraph. The lines also tended to be busy enough to be beyond the capacity of a single track. In the early days theBoard of Trade did not consider any single-track railway line to be complete.
In the earliest days of railways in the United States most lines were built as single-track for reasons of cost, and very inefficient timetable working systems were used to prevent head-on collisions on single lines. This improved with the development of thetelegraph and thetrain order system.
In any given country, rail traffic generally runs to one side of a double-track line, not always the same side as road traffic. Thus inBelgium,China,France (apart from the classic lines of the former GermanAlsace andLorraine),Sweden (apart fromMalmö and further south),Switzerland,Italy andPortugal for example, the railways use left-hand running, while the roads use right-hand running. However, there are many exceptions:
Handedness of traffic can affect locomotive design. For the driver, visibility is usually good from both sides of the driving cab, so the choice of which side the driver should sit is less important. For example, the FrenchSNCF Class BB 7200 is designed to use the left-hand track and therefore uses LHD. When the design was modified for use in the Netherlands asNS Class 1600, the driving cab was not completely redesigned, keeping the driver on the left even though trains use the right-hand track in the Netherlands.[6] Generally, the left/right principle in a country is followed mostly on double track. On steam trains, the steam boiler often obscured some of the view, so the driver was preferably placed nearest to the side of the railway, so that it was easier to see the signals. On single track, when trains meet, the train that does not stop often uses the straight path in the turnout, which can be left or right.[7]
Double-track railways, especially older ones, may use each track exclusively in one direction. This arrangement simplifies thesignalling systems, especially where the signalling is mechanical (e.g.semaphore signals).
Where the signals andpoints (UK term) or rail switches (US) are power-operated, it can be worthwhile to provide signals for each line which cater for movement in either direction, so that the double line becomes a pair of single lines. This allows trains to use one track where the other track is out of service due to track maintenance work, or a train failure, or for a fast train to overtake a slow train.
Mostcrossing loops are not regarded as double-track even though they consist of multiple tracks. If the crossing loop is long enough to hold several trains, and to allow opposing trains to cross without slowing down or stopping, then that may be regarded as double-track. A more modern British term for such a layout is an extended loop.
 | This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(July 2023) (Learn how and when to remove this message) |
The distance between the tracks' centres makes a difference in cost and performance of a double-track line. The track centres can be as closely spaced and as cheap as possible, but maintenance must be done on the side. Signals for bi-directional working cannot be mounted between the tracks, so they must be mounted on the 'wrong' side of the line or on expensivesignal bridges. For standard gauge tracks the distance may be 4 metres (13 ft) or less. Track centres are usually further apart on high speed lines, as pressure waves knock each other as high-speed trains pass. Track centres are also usually further apart on sharp curves, and the length and width of trains is contingent on theminimum railway curve radius of the railway.
Increasing the width of track centres of 6 metres (20 ft) or more makes it much easier to mount signals and overhead wiring structures. Very widely spaced centres at major bridges can have military value.[clarification needed] It also makes it harder for rogue ships and barges to knock out both bridges in the same accident.
Railway lines in desert areas affected by sand dunes are sometimes built with the two tracks separated, so that if one is covered by sand, the other(s) are still serviceable.
If the standard track centre is changed, it can take a very long time for most or all tracks to be brought into line.
On British lines, the space between the two running rails of a single railway track is called the "four foot" (owing to it being 'four foot something' in width), while the space between the different tracks is called the "six foot". It is not safe to stand in the gap between the tracks when trains pass by on both lines, as happened in theBere Ferrers accident of 1917.
When one track of a double-track railway is out of service for maintenance or a train breaks down, all trains may be concentrated on the one usable track. There may be bi-directional signalling and suitable crossovers to enable trains to move onto the other track expeditiously (such as theChannel Tunnel[broken anchor]), or there may be some kind of manual safeworking to control trains on what is now a section of single track.Seesingle-line working.
Accidents can occur if the temporary safeworking system is not implemented properly, as in:
From time to time, railways are asked to transport exceptional loads such as massive electrical transformers that are too tall, too wide or too heavy to operate normally. Special measures must be carefully taken to plan successful and safe operation ofout-of-gauge trains. For example, adjacent tracks of a double line might have to be shut down to avoid collisions with trains on those adjacent tracks.
These are a form of crossing loop, but are long enough to allow trains approaching each other from opposite directions on single-track lines to cross (or pass) each other without reducing speed. In order for passing lanes to operate safely and effectively, trains must be timetabled so that they arrive at and enter the loop with close time tolerances, otherwise they will need to slow or even be brought to a complete stop to allow the oncoming train to pass. They are suited to lines with light to moderate traffic.
An example of where passing lanes have been installed in order to improve travel times and increase line capacity is the 160-kilometre (100-mile) section of theMain Southern railway line in Australia betweenJunee andAlbury. This was built as a single track line in stages between 1878 and 1881, and was partially duplicated between 2005 and 2010 by the construction of four passing lanes each 6 km (4 mi) long. In this instance, this was accomplished by extending pre-existing crossing loops of either 900 metres (3,000 ft) or 1,500 metres (4,900 ft) in length.
The process of expanding a single track to double track is calledduplication ordoubling, unless the expansion is to restore what was previously double track, in which case it is calledredoubling.
The strongest evidence that a line was built as single-track and duplicated at a later date consists of major structures such as bridges and tunnels that are twinned. One example is the twin Slade tunnels on theIlfracombe Branch Line in the UK. Twinned structures may be identical in appearance, or like some tunnels betweenAdelaide andBelair inSouth Australia, substantially different in appearance, being built to differentstructure gauges.
Tunnels are confined spaces and are difficult to duplicate while trains keep on running. Generally they are duplicated by building a second tunnel. An exception is theHoosac Tunnel, which was duplicated by enlarging the bore.
To reduce initial costs of a line that is certain to see heavy traffic in the future, a line may be built as single-track but withearthworks and structures designed for ready duplication. An example is theStrathfield toHamilton line inNew South Wales, which was constructed as mainly single-track in the 1880s, with full duplication completed around 1910. All bridges, tunnels, stations, and earthworks were built for double track. Stations with platforms with 11-foot (3.4 m) centres had to be widened later to 12-foot (3.7 m) centres, except forGosford.
The formerBaltimore and Ohio Railroad (B&O) line betweenBaltimore andJersey City, now owned byCSX andConrail Shared Assets Operations, is an example of a duplication line that was reduced to single-track in most locations, but has since undergone re-duplication in many places between Baltimore andPhiladelphia when CSX increased freight schedules in the late 1990s.
Also:
Some lines are built as single-track with provision for duplication, but the duplication is never carried out. Examples are:
When the capacity of a double-track railway is in excess of requirements, the two tracks may be reduced to one, in order to reduce maintenance costs and property taxes. In some countries this is calledsingling. Notable examples of this in the United Kingdom occurred on the Oxford–Worcester–Hereford, Princes Risborough–Banbury and Salisbury–Exeter main lines during the 1970s and 1980s. In all these cases, increases in traffic from the late 1990s have led to the partial reinstatement of double track. In New Zealand theMelling Line wassingled to theWestern Hutt Railway Station inLower Hutt in 1958 after it became a branch line rather than part of the mainHutt Valley Line.Kirkby railway station (until 1977) andOrmskirk railway station (until 1970) were double-track railway, when they were converted intosingle-track railway withcross-platform interchange.
In New South Wales, Australia, theMain Western Railway betweenWallerawang andTarana, and betweenGresham andNewbridge were singled in the 1990s. A new passing loop was opened on part of the closed track atRydal in the Wallerawang–Tarana section during 2019.[13]
A double-track tunnel with restricted clearances is sometimes singled to form a single track tunnel with more generous clearances, such as theConnaught Tunnel in Canada or the Tickhole Tunnel inNew South Wales, Australia. In the case of the Tickhole Tunnel a new single-track tunnel was built and the two tracks in the original tunnel were replaced by one track in the centreline of the tunnel. Another case where this was necessary was theHastings Line in the United Kingdom, where the tunnels were eventually singled to permit the passage of standardBritish-gauge rolling stock. Before the singling, narrow-bodied stock, specially constructed for the line, had to be used.
As part of theRegional Fast Rail project inVictoria, Australia, the rail line betweenKyneton andBendigo was converted from double- to single-track to provide additional clearance through tunnels and under bridges for trains travelling at up to 160 km/h (99 mph).
A similar process can be followed on narrow bridges (like theBoyne Viaduct, a bridge just north ofDrogheda railway station inIreland).
The bridge over theMurray River betweenAlbury andWodonga is double-track, but because of insufficient strength in the bridge only one train is allowed on it at a time. The bridge has since been singled as part of theNorth East Line Standardisation with the old broad gauge track now disconnected but remains in place on the bridge.
Railways that become especially busy in wartime and are duplicated, especially in World War I, may revert to single track when peace returns and the extra capacity is no longer required. TheFlanders campaign saw duplication of theHazebrouck–Ypres line, amongst other works.
Severe gradients can make theheadway in the uphill direction much worse than the headway in the downhill direction. BetweenWhittingham andMaitland, New South Wales, a third track was opened between Whittingham and Branxton in 2011 and Branxton to Maitland in 2012 to equalize the headway in both directions for heavy coal traffic.[16] Triple track could be a compromise between double-track andquad-track; such a system was proposed south ofStockholm Central Station, but was cancelled in favor ofCitybanan.
InMelbourne andBrisbane several double track lines have a third track signalled in both directions, so that two tracks are available in the peak direction during rush hours.
Triple track is used in some parts of theNew York City Subway and on theNorristown High-Speed Line to add supplemental rush-hour services. The center track, which serves express trains, is signalled in both directions to allow two tracks to be used in the peak direction during rush hours; the outer tracks use bi-directional running and serve local trains exclusively in one direction. During service disruptions on one of the two outer tracks, trains could also bypass the affected sections on the center track.
The Union Pacific Railroad mainline through Nebraska has a 108-mile (174 km) stretch of triple track betweenNorth Platte and Gibbon Junction, due to a high traffic density of 150 trains per day.
Portions of theCanadian National main line in theGreater Toronto Area andSouthern Ontario are triple track to facilitate high traffic density of freight services,intercity, andsuburban passenger trains sharing the same lines.[17]
India, through its state-owned Indian Railways, has initiated the construction of a third track between Jhansi and Nagpur via Bhopal (approximately 590 kilometres (370 miles)) for reducing the traffic load and delays in passenger train arrivals.[18] The construction between Bina and Bhopal[19] and between Itarsi and Budhni had been completed by April 2020.[20]
TheMelbourne toAlbury railway originally consisted of separate1,600 mm (5 ft 3 in) gauge and1,435 mm (4 ft 8+1⁄2 in) gauge single track lines, but when traffic on the broad gauge declined, the lines were converted tobi-directional double track1,435 mm (4 ft 8+1⁄2 in) gauge lines.
| This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(April 2022) (Learn how and when to remove this message) |
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 incommuter rail orrapid transit. The layout can vary, often with the two outer tracks carrying the local trains that stop at every station so one side of stations can be reached without staircase; this can also be reversed, with express trains on the outside and locals on the inside, for example if staffed ticket booths are wanted, allowing one person for both directions. At other places two tracks on one half of the railway carry local trains and the other half faster trains. At the local train stations, the express trains can pass through the station at full speed. For example on theNuremberg-Bamberg railway, which is quadruple track for most of its course, the inner two tracks are used by theS-Bahn Nuremberg whereas the outer tracks are used for regional express andIntercity Express trains. The section in northernFürth where the line is "only" double track creates a major bottleneck. For Berlin Stadtbahn the two northern tracks are local S-Bahn and the two other for faster trains.
One notable example of quadruple track in the United States was thePennsylvania Railroad's main corridor through the heart ofPennsylvania around the famousHorseshoe Curve. This line is now owned by Norfolk Southern. Other examples include theHudson andNew Haven Lines, both of which are shared betweenMetro-North andAmtrak in New York and Connecticut. The New Haven Line is quadruple track along its entire length, while the Hudson Line is only quadruple tracked along the shared portion fromRiverdale toCroton–Harmon and along the shared track fromGrand Central Terminal toYankees–East 153rd Street. Amtrak'sNortheast Corridor is quadruple tracked in most portions south of New Haven, but also has a few triple-track segments. TheMetra Electric District andSouth Shore Line is quadruple-tracked on most of the main line north ofKensington/115th Street station, with local trains running in the center two tracks, and express trains on the outer two tracks. Running parallel are two additional non-electrified tracks that carry freight rail and Amtrak trains, making the entire right of way a total of six tracks.
Outside the United States theChūō Main Line is an example of a modern, heavily utilized urban quadruple track railway.
Quadruple track is used in rapid transit systems as well: throughout theNew York City Subway, theChicago "L"'sNorth Side Main Line, andSEPTA'sBroad Street Line in the United States, and on theLondon Underground in the United Kingdom.
The two tracks of a double-track railway do not have to follow the same alignment if the terrain is difficult. AtFrampton, New South Wales, Australia, the uphill track follows something of a horseshoe curve at 1 in 75 gradient, while the shorter downhill track follows the original single track at 1 in 40 grades.
A similar arrangement to Frampton could not be adopted betweenRydal andSodwalls on theMain Western railway line because the 1 in 75 uphill track is on the wrong side of the 1 in 40 downhill track, so both tracks follow the 1 in 75 grade. Another example is atGunning.
BetweenJunee andMarinna, New South Wales, Australia the two tracks are at different levels, with the original southbound and downhill track following ground level with a steep gradient, while the newer northbound and uphill track has a gentler gradient at the cost of morecut and fill.
At theBethungra Spiral, Australia, the downhill track follows the original short and steep alignment, while the uphill track follows a longer, more easily graded alignment including aspiral.
AtSaunderton, England, what became the London-to-Birmingham main line of theGreat Western Railway in 1909 was initially part of a single-track branch line fromMaidenhead. Down trains follow the route of the old branch line, while up trains follow a more gently graded new construction through a tunnel. This scheme avoided the cost of a new double-track tunnel.
Directional running is two separate lines operationally combined to act as a double-track line by converting each line to unidirectional traffic. An example is in centralNevada, where theWestern Pacific andSouthern Pacific Railroads, longtime rivals who each built and operated tracks betweennorthern California andUtah, agreed to share their lines between meeting points nearWinnemucca andWells, a distance of approximately 180 miles (290 km).[21] Westbound trains from both companies used the Southern Pacific'sOverland Route, and eastbound trains used the Western Pacific'sFeather River Route (now called theCentral Corridor).[22] Crossovers were constructed where the lines ran in close proximity to allow reverse movements. This was necessary as while for most of this run the tracks straddle opposite sides of theHumboldt River, at points the two tracks are several miles apart and some destinations and branch lines can only be accessed from one of the lines. There is a grade separated crossover of the two lines in the shared track area nearPalisade, Nevada, which results in trains followingright hand traffic in the eastern half of the shared track area, butleft hand traffic in the western half. TheUnion Pacific Railroad has since acquired both of these lines, and continues to operate them as separate lines using directional running.Amtrak also runs theCalifornia Zephyr along these routes.[23]
A similar example exists in theFraser Canyon inBritish Columbia, whereCanadian National andCanadian Pacific Kansas City each own a single-track line – often on either side of the river. The companies have a joint arrangement where they share resources and operate the canyon as a double-track line over a 155-mile distance (249 km) between meeting points nearMission andAshcroft.[24] The agreement effectively increased capacity through the corridor from 30 trains per day to over 100 trains daily.[25]
In other cases, where the shared lines already run in close proximity, the two companies may share facilities. InConshohocken, Pennsylvania, where the formerReading Railroad andPennsylvania Railroad shared lines, the lines even shared overhead electrical wire supports, for a 2-mile (3.2 km) stretch on the northern bank of theSchuylkill River. Both lines eventually came underConrail ownership in 1976, with the former PRR line being abandoned and now used as a hiking and bicycle path.[citation needed]
There are about 7,500 miles (12,100 km) of routes operated directionally in the United States and Canada, with about 2,000 miles (3,200 km) of those miles running inTexas.[26]
An unusual example used to exist on theIsle of Wight, where until 1926 parallel tracks betweenSmallbrook Junction andSt John's Road existed. TheSouthern Railway installed the actual junction, but it was only used during heavily trafficked summer months. During the winter, the lines reverted to separate single-track routes.[27]
Because double and single track may use different signalling systems, it may be awkward and confusing to mix double and single track too often. For example, intermediate mechanical signal boxes on a double-track line can be closed during periods of light traffic, but this cannot be done if there is a single-line section in between. This problem is less serious with electrical signalling such asCentralized traffic control.
Media related toDouble-track railway lines at Wikimedia Commons