The Delhi Metro is a rapid transit system serving Delhi, Gurgaon, Noida and Ghaziabad in the National Capital Region of India. The network consists of six lines with a total length of 189.63 kilometres (117.83 mi) with 142 stations of which 35 are underground. It has a combination of elevated, at-grade and underground lines and uses both broad gauge and standard 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 current through 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.

Delhi (/ˈdɛli/; locally pronounced Dillee or Dehli), officially the National Capital Territory of Delhi (NCT), is the largest metropolis by area and the second-largest metropolis by population in India. It is the eighth largest metropolis in the world by population with 16.7 million inhabitants in the Territory at the 2011 Census. The region has been given special status of National Capital Region under The Constitution (69th Amendment) Act, 1991. There are nearly 22.2 million residents in the greater National Capital Region urban area (which also includes the cities Noida, Greater Noida, Ghaziabad, Gurgaon,Sonepat and Faridabad along with other smaller nearby towns).

Delhi is known to have been continuously inhabited since at least the 6th century BCE, though human habitation is believed to have existed since the second millennium BCE. Delhi is also widely believed to have been the site of Indraprastha, the legendary capital of the Pandavas during the times of the Mahabharata. Delhi re-emerged as a major political, cultural and commercial city along the trade routes between northwest India and the Gangetic plain after the rise of the Delhi sultanates. It is the site of many ancient and medieval monuments, archaeological sites and remains.

Mallika Kapur is a Mumbai-based correspondent for CNN.

Kapur is Indian and was born in Calcutta (Kolkata). She graduated in 1997 with a BA (Honours) in International Relations from Brown University, USA, and went on to do a Masters in Broadcast Journalism at Columbia University’s Graduate School of Journalism in New York in 1998.