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Railways can be built above or below ground level, depending on the geography and the topography of the location. Furthermore, trains underneath the ground level might be built at shallow or larger depths.
There are three distinct railway systems:
1. Surface railways
2. Elevated railways
3. Underground railways
1. Surface Railways
With this system, railways are placed above the ground. This is the most widely used rail system on the planet and it’s also among the least expensive. Various mechanisms, such as level crossings, overbridges, and underbridges are provided for crossing the railway, and allow road traffic to flow securely and efficiently across surface railways.
2. Elevated Railways
In this system, railways are placed at a higher or elevated position above the surface. The rail is carried on the steel deck of a continuous viaduct supported on piers, columns, and other structures, usually constructed from steel, cast iron, concrete, or bricks. This system is more expensive than the previous one because stations, waiting rooms, and offices must be built on higher levels, necessitating the usage of staircases, escalators, and other means of transportation.
This form of railway has been used successfully in densely populated urban areas where underground lines are not feasible. The earliest elevated railway was the London and Greenwich Railway, built between 1836 and 1838 on a brick viaduct of more than 800 arches.
3. Underground Railways
In this system, the railway is provided below the ground level. Tunnels are built to transport tracks through them, and an overbridge is required to carry road traffic over railway traffic at every road crossing. Due to ventilation issues in tunnels. electricity is the only source of power available for underground railways.
This technique is suitable for extremely populated areas with high traffic density on roads.
Advantages of underground railways include:
a) Quick and unhindered mobility.
b) Reduction of traffic congestion.
A special type of underground railways is tube railways, which run underground at a depth of 18 meters or more (up to 52 meters).
The segment of the subterranean tunnels carrying the track is circular, like a tube, giving this rail system its name. Tube railways are primarily used to prevent track interference with other railway tracks, water and gas pipes, sewerage systems, and oil or drainage pipelines, among others.
Often, both underground and tube railways are called “rapid-transit systems”, although this term also includes elevated railways.
Geographical barriers, existing or predicted travel patterns, building costs, politics, and historical limits are all variables that influence rapid transit topologies. A rapid-transit system is designed to serve a geographic area with a collection of lines that are described as "I," "U," "S," and "O" forms or loops. Geographical impediments may result in chokepoints where transit lines must converge (for example, to cross a body of water), which can generate congestion but also provide opportunities for line transfers.
Ring lines provide good coverage, connect the radial lines, and serve tangential routes that would otherwise have to travel through the network's normally crowded center.
A grid network can provide a wide range of routes while still maintaining a respectable level of service speed and regularity.