The public sector in modern India And Started Development like Railways, Oil and Coal, Forests Industries, Heavy industries.
In the beginning, these PSUs were limited to those core and tactical industries which would not be touched by private players like Railways, Oil and Coal, Forests Industries, Heavy industries, etc. due to heavy investment, there is a lot of time to complete and humongous risks involved that could lead to a heavy loss if went wrong and also the private players lacked the expertise for the same.
Therefore, the public sector(government) entered the same and started development.
The later or second phase witnessed steps like nationalization of industries, sick units being taken over by private players, and the public sector entering into many other areas like manufacturing consumer goods, consultancy, contracting and transportation etc.
Thus according to Industrial Policies taken out from time to time, there were three categories that industries were classified into with respect to the State's role:
Schedule A category was reserved for the future development of those industries which would be the special responsibility of the State.
Schedule B category included Enterprises whose development initiatives would be driven principally by the State and then private participation would be allowed to supplement the efforts.
Schedule C included remaining industries left to the private sector.