Metro rails belong to a railway system called Rapid Transit System. It is an electric rail system.  Metro trains have unmatched ability to transport large number of people quickly over short distances locally. This largely reduces traffic on roads. Since bigger cities have a lot of traffic, Metro is therefore located in a city or urban area to reduce traffic on road.
However, they are not as fast as long distance trains.
For running of a metro train, rails made of steel are generally put at a height above street level. Thus, metros run above the ground level. Sometimes the rails are also laid in underground tunnels.

After success in Delhi, metros would soon be coming to Chennai. The rails to be laid  are being imported all the way from France through ships. Cranes will be used to place them at that height. After a lot of hard work, they are expected to be operational by December 2013.

Did you know?

Which cities in India have an existing Metro service other than Delhi?  Kolkata, Bangalore