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 prior to the invention of the telegraph. The lines also tended to be busy enough to be beyond the capacity of single track anyway.
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, which is not necessarily the same side that road vehicles in the same country keep to. 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.
Where the French railways which use left hand running meet the German railways which use right hand running, flyovers are provided to convert from one hand to the other.
Many countries do not have a great amount of double track, and the border crossing are usually single track, so the question is moot.
Double track railways, especially older ones, use each track exclusively in one direction. This simplifies the signalling systems, especially where the signalling is mechanical.
Where the signals and points 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. See single-line working for a discussion of this topic.
Crossing loops, while consisting of two or more tracks, are not normally regarded as double track. 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 this precludes maintenance workers standing safely between the lines. 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 4m 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 the work load for the surveyor. If the track centres are increased to 5m or so, this suits high speed trains passing each other, and eliminates the need to widen the centres on sharp curves. If the track centres are increased to 6m or so, signals and overhead wiring structures can always be mounted on the ground.
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.
When one track of a double track railway is out of service for maintenance or a train break 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 for a discussion of this topic.
Accidents can occur if the temporary safeworking system is not implemented properly:
To improve travel times and increase line capacity the 300km of line between Junee and Melbourne is to partially duplicated in a configuration called Passing lanes. Existing crossing loops are mostly 900m and 1500m long, and these will be enhanced by loops 6000m 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.
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).
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. 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.
An unusual stretch of double track in the west of the United States actually comprises two separate single track lines owned by separate companies. However, the capacity of the two tracks is greatly increased if they are combined and operated as if they were a double track line, and this indeed is what the two companies do. 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 Schyulkill 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.
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.