ELECTRIC TRACTION SYSTEM

A railway electrification system supplies electrical energy to railway locomotives and multiple units so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world. Railway electrification has many advantages but requires heavy capital expenditure for installation.

In India 1500 V DC and 25 kV AC, 50 Hz, is used for main line trains.
The 1500 V DC overhead system (negative earth, positive catenary) is used around Mumbai. The Mumbai region is the last bastion of 1500 V DC electrified lines on Indian Railways. There are plans to change this to 25 kV AC by 2010. The 25 kV AC system with overhead lines is used throughout the rest of the country. The dual-voltage WCAM series locomotives haul intercity trains out of Mumbai DC suburban region. The new AC/DC EMU rakes used in Mumbai are also designed to operate with both DC and AC traction as the Mumbai area switches over to the 25 kV AC system.
The Kolkata Metro uses 750 V DC traction with a third rail for delivering the electricity to the EMUs. The Kolkata trams use 550 V DC with overhead lines with underground conductors. The catenary is at a negative potential.
The Delhi Metro uses 25 kV AC overhead lines on the ground-level and elevated routes, and uses a rather unusual "rigid catenary", or overhead power rail, in the underground tunnel sections

Types of traction systems

Electric-traction systems can be broadly divided into those using alternating current and those using direct current. With direct current, the most popular line voltages for overhead wire supply systems have been 1,500 and 3,000. Third-rail systems are predominantly in the 600–750 volt range. The disadvantages of direct current are that
expensive substations are required at frequent intervals and the overhead wire or third rail must be relatively large and heavy. The low-voltage, series-wound, direct-current motor is well suited to railroad traction, being simple to construct and easy to control. Until the late 20th century it was universally employed in electric and diesel-electric traction units.
The potential advantages of using alternating instead of direct current prompted early experiments and applications of this system. With alternating current, especially with relatively high overhead-wire voltages (10,000 volts or above), fewer substations are required, and the lighter overhead current supply wire that can be used correspondingly reduces the weight of structures needed to support it, to the further benefit of capital costs of electrification. In the early decades of high-voltage alternating current electrification, available alternating-current motors were not suitable for operation with alternating current of the standard commercial or industrial frequencies (50 hertz [cycles per second] in Europe; 60 hertz in the United States and parts of Japan). It was necessary to use a lower frequency (16 ²/₃ hertz is common in Europe; 25 hertz in the United States); this in turn required either special railroad power plants to generate alternating current at the required frequency or frequency-conversion equipment to change the available commercial frequency into the railroad frequency.

Characteristics of electric traction

The main advantage of electric traction is a higher power-to-weight ratio than forms of traction such as diesel or steam that generate power on board. Electricity enables faster acceleration and higher tractive effort on steep grades. On locomotives equipped with regenerative brakes, descending grades require very little use of air brakes as the locomotive's traction motors become generators sending current back into the supply system and/or on-board resistors, which convert the excess energy to heat.
Other advantages include the lack of exhaust fumes at point of use, less noise and lower maintenance requirements of the traction units. Given sufficient traffic density, electric trains produce less carbon emissions than diesel trains, especially in countries where electricity comes primarily from non-fossil sources.
The main disadvantage is the capital cost of the electrification equipment, most significantly for long distance lines which do not generate heavy traffic. Suburban railways with closely-spaced stations and high traffic density are the most likely to be electrified, and main lines carrying heavy and frequent traffic are also electrified in many countries.

• Current
• Direct current (DC)
• Alternating current (AC)

• Contact System
• third rail
• overhead line (catenary)

Overhead systems

Overhead lines

1.5 kV DC is used in the Netherlands, Japan, Ireland, Australia (parts), India, France, New Zealand and the United States. In Slovakia, there are two narrow-gauge lines in the High Tatras . In Portugal, it is used in the Cascais Line, and in Denmark on the suburban S-train system.
loops at one end, causing the train to be reversed during every complete journey 

Advantages and disadvantages

Advantages include:

• lower running cost of locomotives and multiple units
• lower maintenance cost of locomotives and multiple units
• higher power-to-weight ratio, resulting in
• fewer locomotives
• faster acceleration
• higher practical limit of power
• higher limit of speed
• less noise pollution 
• reduced power loss at higher altitudes 
• lack of dependence on crude oil as fuel

Disadvantages include:

• upgrading brings significant cost,
• especially where tunnels and bridges and other obstructions have to be altered for clearance
• alterations or upgrades will be needed on the railway signaling to take advantage of the new traffic characteristics

Energy efficiency

There is a significant amount of published material that concludes that electric trains are more energy efficient than diesel-powered trains, and with proper energy production can have a smaller carbon dioxide footprint. Some of the reasons for this are given below:

• electric trains may be powered from a number of different sources of energy  as opposed to diesel trains that are reliant on oil.
• under certain conditions  trains can return power to the network , further increasing efficiency.
• electric trains do not have to carry around the weight of their fuel unlike diesel traction.

In order for trains to return power to the network, both the rolling stock and the network must be prepared to do so. Presently the energy returned by vehicles is not sent back to the public network, but made available for other vehicles within the network. Regenerative braking is therefore often implemented in tram networks, where the density of vehicles per powered section is high, but is more difficult with trains, especially where the voltage is relatively low, hence the sections are small.
According to widely accepted global energy reserve statistics  the reserves of liquid fuel are much less than gas and coal (at 42, 167 and 416 years respectively). And most countries with large rail networks do not have significant oil reserves, and those that do, like the United States and Britain, have exhausted much of their reserves and have had declining oil output for decades. Therefore there is also a strong economic insentive to substitute oil for other fuels.