Double track

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 were provided so that train moving on the left in France end up on the right in Germany and vice versa.

Handedness of traffic can affect locomotive design. For the driver, visibility is good from both sides of the driving cab so the choice on which side to site the driver less important. For example, the French SNCF Class BB 7200 is designed for using 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 despite the fact that trains use the right-hand track in the Netherlands. Generally, the left/right principle in a country is followed mostly on double track. On single track, when trains meet, the train that shall not stop often uses the straight path in the turnout, which can be left or right.

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).

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Wikipedia