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.
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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 prior to the invention of the telegraph. The lines also tended to be busy enough to be beyond the capacity of 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 employed 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 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.
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.
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.
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.
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.
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.
(put in order of size)
If the standard track centres is changed, it can of course 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" (same reason). 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
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 onto 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:
To improve travel times and increase line capacity the 300 kilometres (190 mi) of line between Junee and Melbourne is to 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.
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 United Kingdom. Twinned structure may be identical in appearance, or like some tunnels between Adelaide and Belair South Australia, substantially different in appearance.
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 would be the Hoosac 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 with earthworks and structures designed for ready duplication. An example is the Strathfield to Hamilton line in New South Wales Australia which was constructed as mainly single track in the 1880s, with full duplication only completed around 1910. All bridges, tunnels, stations, and earthworks were built for double track. 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 duplication in many places between Baltimore and Philadelphia, Pennsylvania when CSX increased freight schedules in the late 1990s.
Also Smardale Gill viaduct.
Some lines are built as single track with provision for duplication, but the duplication is never carried out. Examples include:
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.
A double track tunnel with restricted clearances is sometimes singled to form a single track tunnel with 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. A notorious 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 (prior to the singling, narrow bodied stock was used, specially constructed for the line).
As part of the Regional Fast Rail (RFR) railway upgrade 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.
A similar process can be followed on narrow bridges (like the bridge north of Drogheda railway station in Ireland).
The bridge over the Murray River between Albury and Wodonga is double track, but because of weakness in the bridge only one train is allowed on it at a time.
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 were cancelled in favor to Citybanan.
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 both local trains that stop often, and also faster inter-city or high speed trains.
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 cannot have been 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 having a gentler gradient at the cost of more cut and fill.
At Bethungra, New South Wales, Australia, the downhill track follows the original short and steep alignment, while the uphill track follows a longer, more easily graded alignment including a spiral.
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 is when two tracks that were built as separate lines are operationally combined to act as a double track line. 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).[5] 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).[6]
Where the lines ran in close proximity, crossovers were constructed 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.[7]
Canadian Pacific and Canadian National are starting to do the same. Another example was in Conshohocken, Pennsylvania where the former Reading Railroad and Pennsylvania Railroad shared lines, and even 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.[8]
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.
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