Why don't indian railways use gps system to run trains in dense fog conditions.?

Same reason they dont use GPS here, it isnt accurate enough. A GPS cant always tell if a train is on a track or the one immediatly beside it, for instance a main track and the side track 8 feet away. And GPS is even less reliable in dense for or heavy storms than in clear weather. In the US they primarily use a system of in track sensors and signals to keep trains separated. There is GPS in many locomotives but it is not used as the primary means of collision avoidance.

GPS is the property of the US military. It always works at full strength for the military because they always send an ecrypted signal with the correct data that only the military can decrypt. For civilians (unencrypted), it has three modes. 1 Full accuracy. 2 Fuzzy, meaning it gives vague information accurate within 1000 feet or so. 3. OFF, meaning civilians cannot use it at all. Only US military encrypted receivers. They would turn it down or off, to keep enemies from using the system against the U.S. Now, what do your trains do????? Stop??? Your whole railroad shut down because some lame country developed a GPS guided cruise missile? Great.