Yeah, as Arunava said, the Mumbai Metro is currently under construction. It's comprised of both subway and light rail. Here's the first few phases shown with main nodes:
More detailed map of line 2:
The stations will be developed and run by private developers who will also use them as malls and transportation nodes. An example station:
Really, there's a massive upgrading of Mumbai's infrastructure underway. Apart from the Metro, there are upgrades in the works for the rail system to increase capacity, revamping train stations, expansion and modernization of bus lines, stations and grid, major highway/flyover projects to decrease congestion and cut travel time, the construction of up to two more international airports, as well as the modernization of the Chhatrapati Shivaji International Airport
You can check out the Mumbai -- Project News and Developments
thread in the India section for a a more detailed rundown.