A double-track railway usually involves running one track in each direction, compared to a single-track railway, where trains in both directions share the same track.
Overview
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 the telegraph. The lines also tended to be busy enough to be beyond the capacity of a single track. In the early days, the Board of Trade did not consider any single-track railway line to be complete.
In the United States, most lines were built as single-track for reasons of cost; very inefficient timetable working systems were used to prevent head-on collisions on single lines. This improved with the development of the telegraph and the train order system.
Operation Handedness
In any given country, railway traffic generally runs to one side of a double-track line, not always the same side as road traffic. Thus in Belgium, China, France, Sweden, Switzerland, Italy and Portugal for example, the railways use left-hand running, while the roads use right-hand running.
However, there are many exceptions: Metro systems often are RHT, even when the heavy rail network runs on the left. In Finland, railway traffic uses mainly right, with the exception of the Ring Rail Line and the separate commuter rail tracks to Kerava and Leppävaara in Helsinki area which use the left. In Switzerland, the Lausanne Metro and railways in the Germany border area, as well as all tram systems, use RHT. In Indonesia, right-hand running for railways (including LRT and MRT systems) and left-hand running for roads. In Spain, where rails are RHT, metro systems in Madrid and Bilbao use LHT.Moya, Aurora. "Metro de Madrid, 1919–1989. Setenta años de historia", Chapter 1 In Portugal, all heavy rail systems, including the national railway network, high speed lines and the Lisbon Metro use LHT; while all light rail systems use RHT because the majority of them share their tracks with the road. In Sweden, the tram systems in Gothenburg (except for Angered station and stations with island platforms), Lund, Norrköping and Stockholm (except the Alvik – Alleparken section) use the right. The railways and the metro use the left in general, but in Malmö they use the right due to the connection to Denmark. In Ukraine, some sections of Kryvyi Rih Metrotram use the left due to tramcars having doors only on right side, which makes it impossible to use the right at stations with island platforms. On the pre-1918 French-German border, for example, flyovers were provided so that trains moving on the left in France end up on the right in Germany and vice versa. In the United States, three Metra commuter railroad lines formerly owned by the CNW near Chicago operate with left-hand running, a historical oddity caused by the original placement of station buildings and the directionality of travel demand.
Locomotive design
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 French SNCF 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 as NS 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. Generally, the left/right principle in a country is followed mostly on double track.
Steam locomotives could be driven from either side, railway companies generally made a decision about which side of the cab the driver would operate the controls from. The UK's Great Western Railway used the left-hand track but their drivers were positioned on the right-hand side of the locomotives, in contrast the UK's Lancashire and Yorkshire Railway, also used the left-hand track but positioned their drivers on the left.
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."Die Functionsweise der Apparaturen auf den automatischen Kreuzungsstationen ist so programmiert, dass den Zügen immer freie Fahrt über das Stationsgleis mit der höheren V max signalisiert wird, sofern der vorausliegende Abschnitt frei ist."
Bi-directional running Double-track railways, especially older ones, may use each track exclusively in one direction. This arrangement simplifies the signalling systems, especially where the signalling is mechanical (e.g. semaphore signals).
Where the signals and points (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.
Crossing loops Most crossing 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.
Track centres
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 expensive signal bridges. For standard gauge tracks, the distance may be 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 the minimum railway curve radius of the railway.
Increasing the width of track centres of or more makes it much easier to mount signals and overhead wiring structures. Very widely spaced centres at major bridges can have military value. 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.
Accidents 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 the Bere Ferrers accident of 1917.
Narrow track centres on the Liverpool and Manchester Railway contributed to a fatal accident on opening day. A US naval scientist and submarine pioneer, Captain Jacques, was killed getting out of the wrong side of a train at Hadley Wood in 1916. Narrow track centres contribute to "Second Train Coming" accidents at level crossings since it is harder to see the second train – for example, the accident at Elsenham level crossing in 2005.
Temporary single track
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 in the Channel Tunnel), or there may be some kind of manual safeworking to control trains on what is now a section of single track.
Accidents can occur if the temporary safeworking system is not implemented properly, as in:
Brühl train disaster, Germany – 2000 Zoufftgen rail crash, France and Luxembourg – 2006.
Out-of-gauge trains 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 of out-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.
Passing lanes 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 section of the Main Southern railway line in Australia between Junee and Albury. 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 long. In this instance, this was accomplished by extending pre-existing crossing loops of either or in length.
Construction Duplication The process of expanding a single track to double track is called duplication or doubling, unless the expansion is to restore what was previously double track, in which case it is called redoubling.
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 the Ilfracombe Branch Line in the UK. Twinned structures may be identical in appearance, or like some tunnels between Adelaide and Belair in South Australia, substantially different in appearance, being built to different structure gauges.
Tunnel duplication 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 the Hoosac Tunnel, which was duplicated by enlarging the bore.
Carried-out provision for duplication
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 with earthworks and structures designed for ready duplication. An example is the Strathfield to Hamilton line in New 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 centres had to be widened later to centres, except for Gosford.
The former Baltimore and Ohio Railroad (B&O) line between Baltimore and Jersey City, now owned by CSX and Conrail 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 and Philadelphia when CSX increased freight schedules in the late 1990s.
Also: Smardale Gill viaduct. Westerham line. The Menangle Bridge was single track (1863) but built for double track (1890s). A second track was laid temporarily to allow testing both tracks at once. The Long Island Rail Road's Ronkonkoma Branch was originally single track for most of its length, but land for a double track was purchased as part of a project to electrify part of the line in the 1980s. A double track was laid along the rest of the segment in 2018.