Double-track railway

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.

Track centres are usually further apart on high speed lines, as pressure waves knock each other as high-speed trains pass.

Increasing the width of track centres of 6 metres (20 ft) or more makes it much easier to mount signals and overhead wiring structures.

[clarification needed] It also makes it harder for rogue ships and barges to knock out both bridges in the same accident.

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.

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.

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 6 km (4 mi) long.

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.

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.

In all these cases, increases in traffic from the late 1990s have led to the partial reinstatement of double track.

In New South Wales, Australia, the Main Western Railway between Wallerawang and Tarana, and between Gresham and Newbridge were singled in the 1990s.

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.

Railways that become especially busy in wartime and are duplicated, especially in World War I, may revert to single track when peace returns and the extra capacity is no longer required.

[16] 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 and on the Norristown High-Speed Line to add supplemental rush-hour services.

The Union Pacific Railroad mainline through Nebraska has a 108-mile (174 km) stretch of triple track between North Platte and Gibbon Junction, due to a high traffic density of 150 trains per day.

Portions of the Canadian National main line in the Greater Toronto Area and Southern Ontario are triple track to facilitate high traffic density of freight services, intercity, and suburban passenger trains sharing the same lines.

[17] India, through its state-owned Indian Railways, has initiated the construction of a third track between Jhansi and Nagpur via Bhopal (approximately 590 kilometres (370 miles)) for reducing the traffic load and delays in passenger train arrivals.

The layout can vary, often with the two outer tracks carrying the local trains that stop at every station so one side of stations can be reached without staircase; this can also be reversed, with express trains on the outside and locals on the inside, for example if staffed ticket booths are wanted, allowing one person for both directions.

Other examples include the Hudson and New Haven Lines, both of which are shared between Metro-North and Amtrak in New York and Connecticut.

Amtrak's Northeast Corridor is quadruple tracked in most portions south of New Haven, but also has a few triple-track segments.

Outside the United States the Chūō Main Line is an example of a modern, heavily utilized urban quadruple track railway.

Quadruple track is used in rapid transit systems as well: throughout the New York City Subway, the Chicago "L"'s North Side Main Line, and SEPTA's Broad Street Line in the United States, and on the London Underground in the United Kingdom.

An example is in central Nevada, where 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).

There is a grade separated crossover of the two lines in the shared track area near Palisade, Nevada, which results in trains following right hand traffic in the eastern half of the shared track area, but left hand traffic in the western half.

The companies have a joint arrangement where they share resources and operate the canyon as a double-track line between meeting points near Mission and Ashcroft.

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.