China's state railways company is pushing NSW to redesign its $7 billion second Sydney Harbour rail crossing to accommodate a second deck for a high-speed rail line.

Patrick Yu, chief executive of Centurion Group, a developer which is China Rail's local partner, said that it would cost only $250 million more to build one big 16-metre diameter tunnel with two decks instead of the current plan for two single-deck six metre tunnels to carry suburban trains for the new Sydney Metro.

He said that even though a high-speed rail line has not yet been approved it still made sense to build a bigger tunnel just in case. He said China would consider making up the difference in cost. "Another harbour rail tunnel alignment would be extremely difficult to secure and it would take 10 years. It would be a crying pity not to have future-proofed infrastructure."

Rail freight is one of Premier Li Keqiang's favoured economic indicators. Fairfax

The China Rail consortium has been pushing for a $25 billion 150-kilometre high-speed rail line from Newcastle north to Campbelltown to the south-west for some years but it has recently secured the support of Fred Nile, leader of the Christian Democrats which controls the balance of power in the NSW upper house.

Reverend Nile said he had written to NSW Premier Mike Baird asking him to consider altering the design of the rail tunnel. "I think the government should make the decision straight away to cater for high-speed rail," Mr Nile said.