The Delhi Metro is a rapid transit system serving Delhi, Gurgaon, Noida and Ghaziabad in the National Capital Region of India. It is one of the largest metro networks in the world. The network consists of six lines with a total length of 189.63 kilometres (117.83 mi) with 142stations of which 35 are underground. It has a combination of elevated, at-grade and underground lines and uses both broad gauge andstandard gauge rolling stock. Four types of rolling stock are used: Mitsubishi-ROTEM Broad gauge, Bombardier MOVIA, Mitsubishi-ROTEM Standard gauge and CAF Beasain Standard gauge.
Delhi Metro is being built and operated by the Delhi Metro Rail Corporation Limited (DMRC). As of November 2010, DMRC operates around 2,700 trips daily between 6:00 and 23:00 running with an interval of 2 minutes 30 seconds between trains at peak frequency.The trains are mainly of four coaches, but due to increase in passengers numbers, six-coach trains are also added on the Red Line (Dilshad Garden to Rithala), Yellow Line (Jahangirpuri to HUDA city centre), Blue Line (Dwarka sec −21 to Vaishali/NOIDA city centre) and Violet Line (Central Secretariat to Badarpur). The power output is supplied by 25-kilovolt, 50 Hertz alternating currentthrough overhead catenary. The metro has an average daily ridership of 1.8 million commuters, and, as of July 2011, had carried over 1.25 billion commuters since its inception. The Delhi Metro Rail Corporation has been certified by the United Nations as the first metro rail and rail-based system in the world to get “carbon credits for reducing greenhouse gas emissions” and helping in reducing pollution levels in the city by 630,000 tons (630 Gg) every year.
Planning for the metro started in 1984, when the Delhi Development Authority and the Urban Arts Commission came up with a proposal for developing a multi-modal transport system for the city. The Government of India and the Government of Delhi jointly set up the Delhi Metro Rail Corporation (DMRC) in 1995. Construction started in 1998, and the first section, on the Red Line, opened in 2002, followed by the Yellow Line in 2004, the Blue Line in 2005, its branch line in 2009, the Green and Violet Lines in 2010 and the Delhi Airport Metro Express in 2011.