I think that amount should be enough to build a new planned city rather than build a railway system. That way Kolkata would be somewhat relieved of its already burdened infrastructure. Another city about 100 kms away from Kolkata that has proper transportation with Kolkata would do a world of good for West Bengal. It shall ensure that a percentage of the population move away from Kolkata, easying the city. Provide a second destination for business, medical, education, shoping etc. to grow. Being 100 kms off Kolkata would ensure the existing facilities (airports, rail etc) are still accessiable. Also ease out some of the real estate, traffic and parking issues. This rail system will help the people associated with the companies situated in the Salt lake area and the now upcoming Rajarhat areas, but i think a more value add would be to spend that amount in developing a new city and expanding the oppourtunities giving the future generation a second destination to choose from.
