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.
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 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 the telegraph and the train 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 in Belgium, China, France (apart from the former German Alsace and Lorraine), Sweden, Switzerland and Italy for example, the railways use left-hand running, while the roads use right-hand running. In countries such as Indonesia, it is the reverse (right-hand running for railways and left-hand running for roads).
On the (pre-1918) French-German border, for example, flyovers are provided so that train moving on the left in France end up on the right in Germany and vice versa.
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 signal each line in both directions, 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 track centres makes a difference in cost and performance of a double-track line. The track centres can be as narrow 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 must be mounted on the 'wrong' side of the line or on expensive signal bridges. Very narrow track centres are also undesirable for high speeds, as pressure waves knock each other as high-speed trains pass.
Narrow track centres might be 4 metres (13 ft) or less. Narrow track centres may have to be widened on sharp curves to allow for long rail vehicles following the arc of the curve, and this increases a surveyor's workload. Widening a track centre to 5 metres (16 ft) or so suits high-speed trains passing each other, and eliminates the need to widen the centres on sharp curves. Increasing width of track centres of 6 metres (20 ft) or more makes it much easier to mount signals and overhead wiring structures.
In the United Kingdom the early lines of the Great Western Railway were built to Brunel's broad gauge, which was also associated with a more generous loading gauge. This is still apparent along those lines which remain in use today: structures such as bridges and tunnels are larger, and the distances between opposite platform faces are larger.
Very wide centres at major bridges can have military value. It also makes it harder for rogue ships and barges knocking out both bridges in the same accident.
Railway lines in desert areas affected by sand dunes are sometimes built on alternate routes so that if one is covered by sand, the other(s) are still serviceable.
Track centres are usually wider on high speed lines to reduce pressure pulse when trains pass each other in opposite directions.
Track centre examples 
(put in order of size)
- 09 ft 8.0 in (2.95 m) Liverpool and Manchester Railway 1830 at opening day; later widened.
- 10 ft 8.5 in (3.26 m) United Kingdom (a six foot plus a four foot)
- 11 ft 0.0 in (3.35 m) New South Wales 1855 old standard (estimated)
- 12 ft 0.0 in (3.66 m) New South Wales 1910 new standard
- 13 ft 1.5 in (4.00 m) New South Wales 1973 metric standard
- 14 ft 9.0 in (4.50 m) Certain ARTC lines after about 2012. 
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 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 good track. There may be bi-directional signalling and suitable crossovers to enable trains to move on to the other track expeditiously (see, for example, the article on the Channel Tunnel), or there may be some kind of manual safeworking to control trains on what is now a section of single track. See single-line working.
Accidents can occur if the temporary safeworking system is not implemented properly:
Out of gauge trains 
From time to time, railways are asked to transort 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 
To improve travel times and increase line capacity, the 300 kilometres (190 mi) of line between Junee and Melbourne is to be partially duplicated in a configuration called Passing lanes. Existing crossing loops are mostly 900 metres (3,000 ft) and 1,500 metres (4,900 ft) long, and these will be enhanced by loops 6 kilometres (3.7 mi) long which are long enough to be regarded as nearly double track.
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 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.
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 11' centres had to be widened later to 12' 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, Pennsylvania when CSX increased freight schedules in the late 1990s.
- 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. 
Never-used provision for duplication 
Some lines are built as single-track with provision for duplication, but the duplication is never carried out. Examples include:
- Swanage Railway
- Bluebell Railway: Horsted Keynes to Lewes.
- Keighley and Worth Valley Railway - including bridge and tunnels and a deviation.
- Westerham in Kent.
- Mid-Wales Railway (parts).
- Neath and Brecon Railway
- Monkerei Tunnel in New South Wales - double track size at summit reduces fume problem in tunnel.
- Golgotha Tunnel near Eythorne on the East Kent Light Railway was only partly excavated for the second track.
- Skitube Alpine Railway - A provision for a second 300-metre-long passing loop has been made inside the tunnel.
When the capacity of a double-track railway is in excess of requirements, the two tracks may be reduced to one. In some countries this is called singling. 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.
Tunnel singling 
A double-track tunnel with restricted clearances is sometimes singled to form a single track tunnel with more generous clearances, such as the Connaught Tunnel in Canada or the Tickhole Tunnel in New 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 the Hastings Line in the United Kingdom, where the tunnels were eventually singled to permit the passage of standard British-gauge rolling stock. Before the singling, narrow-bodied stock, specially constructed for the line, had to be used.
As part of the Regional Fast Rail project in Victoria, Australia, the rail line between Kyneton and Bendigo 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).
Other tunnel singling 
- Hoosac Tunnel, Guilford Rail System, Massachusetts, US.
- Old Main Line Subdivision, B&O, Maryland, US. Entire 58 miles (93 km) subdivision was single-tracked to utilize higher clearances of the 9 tunnels on the line.
- Whitehall Tunnel, B&O, Pennsylvania, US.
Wartime doubling 
Railways that become especially busy in wartime and are duplicated, especially in World War I, may revert to single track when peace arises and the extra capacity is no longer required. The Flanders campaign saw duplication of the Hazebrouck–Ypres line, amongst other works.
Triple track 
Severe gradients made the headway in the uphill direction much worse than the headway in the downhill direction. Between Whittingham and Maitland, a third track is being provided to equalize the headway in both directions for heavy coal traffic. Triple track could be a compromise between double-track and quad-track; such a system was proposed south of Stockholm Central Station, but was cancelled in favor of Citybanan.
Triple track is used in some parts of the New York City Subway to supplement rush-hour service on some lines.
Dual gauge 
The Melbourne to Albury railway originally consisted of separate 1,600 mm (5 ft 3 in) gauge and 1,435 mm (4 ft 8 1⁄2 in) gauge single track lines, but when traffic on the broad gauge declined, the lines were converted to bi-directional double track 1,435 mm (4 ft 8 1⁄2 in) gauge lines.
Quadruple track 
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 trains. The most notable example of quadruple track in the United States, and perhaps the only four-track section of mainline therein, was the Pennsylvania Railroad's main corridor through the heart of Pennsylvania around the famous Horseshoe Curve. This line is now owned by Norfolk Southern. Other examples include the Hudson and New Haven Lines, both of which are shared between Metro-North and Amtrak in New York and Connecticut. The New Haven Line is quadruple track along its entire length, while the Hudson Line is only quadruple track along the shared portion from Spuyten Duyvil to Croton-Harmon and along the length from Grand Central to Yankees-E 153 St.
Non-parallel double track 
The two tracks of a double-track railway do not have to follow the same alignment if the terrain is difficult. At Frampton, 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 between Rydal and Sodwalls 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 at Gunning.
Between Junee and Marina, 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 more cut and fill.
At Saunderton, United Kingdom, what became the London-to-Birmingham main line of the Great Western Railway in 1909 was initially part of a single-track branch line from Maidenhead. 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 
Directional running is when two separate lines are operationally combined to act as a double-track line by converting each line to unidirectional traffic. An example of directional running occurs in central Nevada. The Western Pacific and Southern Pacific Railroads, longtime rivals who each built and operated tracks between northern California and Utah, agreed to share their lines between meeting points near Winnemucca and Wells, a distance of approximately 180 miles (290 km). Westbound trains from both companies used the Southern Pacific's Overland Route while eastbound trains used the Western Pacific's Feather River Route (now called the Central Corridor).
In the Nevada example, crossovers were constructed where the lines ran in close proximity to allow reverse movements. This was necessary as at points the two tracks are several miles apart and some destinations can be only be accessed from one of the lines. The Union Pacific Railroad has since acquired both of these lines; however, continues to operate them as separate lines in a directional running setup.
In other cases, where the shared lines already run in close proximity, the two companies may share facilities. In Conshohocken, Pennsylvania, where the former Reading Railroad and Pennsylvania Railroad shared lines, the lines even shared overhead electrical wire supports, for a 2-mile stretch on the northern bank of the Schuylkill River. Both lines eventually came under Conrail ownership in 1976, with the former PRR line being abandoned and now used as a hike/bike path.
An unusual example used to exist on the Isle of Wight, where until 1926 parallel tracks between Smallbrook Junction and St John's Road existed. The Southern 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.
Mixing double and single track 
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 as Centralized traffic control.
- "Pioneer of Submarines.". The Argus (Melbourne). 29 November 1916. p. 8. Retrieved 30 June 2011.
- "OPENING OF THE RAILWAY TO PICTON.". The Sydney Morning Herald (NSW : 1842 - 1954) (NSW: National Library of Australia). 21 July 1863. p. 8. Retrieved 9 September 2012.
- Soloman, Brian (2003). Railway Masterpieces. Newton Abbot, Devon, UK: David & Charles. p. 122. ISBN 0-87349-323-0.
- Harwood, Jr., Herbert H. (1979). Impossible Challenge: The Baltimore & Ohio Railroad in Maryland. Baltimore, MD: Barnard, Roberts. p. 174. ISBN 0-934118-17-5.
- "Eureka County, Yucca Mountain Existing Transportation Corridor Study". Eureka County – Yucca Mountain Project. 2005. Retrieved 2010-05-08.
- Benchmark Maps (2003). Nevada Road and Recreation Atlas (Map). 1:250000 (2003 ed.). p. 41–44. ISBN 0-929591-81-X. http://www.benchmarkmaps.com.
- "Eureka County, Yucca Mountain Existing Transportation Corridor Study". Eureka County – Yucca Mountain Project. 2005. Retrieved 2010-05-08.
- Trains November 2009, p46
- Hardy, Brian (2003). Tube Trains on the Isle of Wight. Harrow Weald, Middlesex: Capital Transport. p. 8. ISBN 1-85414-276-3.
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