Well, there are two major types as I said earlier: "local" or suburban trains that runs a short distance (<100 km). They are not air-conned. Other type is "express" or inter-city trains that run long-distance. Most of them have one or more compartments with air-conditioning. Some trains have all compartments air-conned, but they are pretty less in number. Generally you will see 10-30% compartments air-conned, but the number varies pretty widely depending on the particular train. The logic behind such a system is AC tickets cost more, so poor people can afford and rich people can get better amenities.