HUNDREDS of extra trains could be rolling across Sydney Harbour every day using a new track hidden beneath the Harbour Bridge.

A team of transport experts, headed by the state's former rail and roads boss Ron Christie, has recommended the government investigate a proposal to build a deck suspended from the bridge undercarriage.

The deck would be barely visible, built inside the existing struts and beams, and is vital to solving congestion on CityRail lines. The ''thought provoker'' is part of an independent inquiry into public transport, backed by The Sun-Herald and The Sydney Morning Herald.

''We absolutely think it's essential to get a second crossing in the near future,'' Mr Christie said. ''Unless there is a further route through the CBD, the rail system is going to be permanently limited in its capacity.''

A second rail line over the harbour has long been identified as critical to increasing the number of train services throughout the CityRail network. The current maximum capacity on the Harbour Bridge is 20 services an hour each way.

But the question of where to build the new line has proven difficult and costly. An earlier proposal to hang a road deck under the bridge was slammed as an eyesore.

In 2005, premier Bob Carr announced plans for a tunnel under the harbour floor, to be built by 2017. It would cost $5 billion.

The Roads and Traffic Authority has also resisted suggestions it should give up lanes on the bridge to accommodate more train tracks.

''The state government seems to find the harbour crossing very difficult and it seems to have walked away from the idea,'' Mr Christie said.

The government further complicated the problem by saying it would build a seven-kilometre, $5 billion metro line from Central to Rozelle, using a corridor under Pitt Street.

Mr Christie's team says that corridor is vital for a new heavy rail line and in its report accuses the Sydney Metro Authority of staging a ''land grab''.

If the authority gets its way, the inquiry team warns that the government's earlier, and favoured plan, for a CityRail line under Pitt Street, with a tunnel under the harbour, would become impossible. That would leave the proposal for a second deck under the bridge as the only option available to serve the busiest parts of CBD.

In a submission to the inquiry, Australian Infrastructure Solutions, says the second, rail-only deck would not stop large ships from passing under the bridge, as it would lower the height by only 1.5 metres.

The proposal also includes:

A new deep underground central route through the CBD.

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

A new direct underground route from the Harbour Bridge to St Leonards via North Sydney and Crows Nest.

The new network would link the Airport and North Shore lines, providing a direct route between the airport and the CBD, the lower north shore, Chatswood and Macquarie Park, which form an ''economic arc'', rich in jobs.

Before rubber-stamping the double-deck bridge concept, the inquiry urged ''immediate, serious and independent investigation'' of the Australian Infrastructure Solutions plan. ''It looks like a less expensive option [than a tunnel] and, for that reason, if no other, it needs to be pursued and investigated,'' Mr Christie said.