A new hidden track beneath the Harbour Bridge could vastly increase the trains to cross the iconic structure.

This is the finding of a preliminary report into a 30-year transport plan for Sydney, created by an independent exert team headed by former NSW road and rail boss, Ron Christie.

The report, which was released on Saturday for final public review, focuses on finding a new route down the central spine of the CBD and avoiding the “land grab” executed by the CBD Metro.

The report argues that it would be possible to construct a lower railway-only deck almost entirely within the bridge’s existing structural supports and under the existing road and rail deck.

This means that trains would be hidden from view without impacting upon the movements of large ships.

A new Harbour Bridge rail crossing would also be significantly cheaper than construction long rail tunnels under some of the deepest parts of the harbour.

The inquiry also proposes a new deep rail route through the CBD. The lines would be deep enough to avoid building basements, the CBD Metro and the Cross City Tunnel and linked to the existing rail network of Central Station.

New stations under Castlereagh Street and Pitt Street mall and new platforms at Wynyard.

Disused train platforms 26 and 27 at Central Station should also be used as part of a new central route through the CBD that would link the airport directly with the Lower North Shore, Macquarie Park and beyond.

The cross-CBD, cross-Harbour heavy rail link could bring a “very large” 25 per cent increase in the capacity of the railway running into and through the CBD.

Christie has called for further investigation, saying the city can not afford to overlook a proposal that, if viable, could “simultaneously overcome both the cost and “CBD Metro” barriers to a much higher capacity CityRail network”.