A second harbour rail crossing and three new underground train stations will be built if the government sells part of the state's electricity network, Premier Mike Baird says.

An estimated $20 billion flowing from the sale would be used to fund a suite of major infrastructure projects, under a policy the NSW government will put to voters at next year's state election.

Among the projects that would be funded by the sale are a new Sydney rapid transit line, including a second rail crossing for Sydney Harbour, and extending the north-west rail link through the Sydney CBD to Bankstown.