TRAIN services into central Sydney would be shut for months and restricted for years under plans by Infrastructure NSW to avoid building a second rail crossing over Sydney Harbour.

That is according to analysis by Transport for NSW which, for half a decade, has been trying to avoid the cost of the crossing estimated at $10 billion.

The shutdown, which would affect the daily commute of tens of thousands of workers, would be needed under plans to upgrade stations in the central business district and track infrastructure. The objective would be to run up to 30 single-deck trains an hour instead of the 20 double-deckers it can run now.

The department and the Transport Minister, Gladys Berejiklian, rejected this idea only in May after deciding the disruption would not be worth the benefit.

Train commuters to the CBD would need to be dropped off on either side of the city – at Redfern, Chatswood or North Sydney – and taken by bus to the city. A limited service would remain for years.

The idea was revived last week as part of Infrastructure NSW’s 20-year strategy. Infrastructure NSW, set up as an independent adviser to the government, disputes the analysis. It says its job is to challenge a bias in Transport for NSW towards new infrastructure such as another harbour crossing.

”The general focus of the NSW transport bureaucracy over a very long time has been about building stuff,” the chairman of Infrastructure NSW, Nick Greiner, said last week. He wants to eke more out of the existing network. ”No matter where you come out you cannot believe that the existing thing is run anywhere near capacity,” he said.

Mr Greiner’s plan rejected the idea of adding to the city’s train system in the next two decades, beyond the north-west and south-west rail links.

Instead of a second harbour crossing, which Transport for NSW now says is necessary, the strategy recommends spending $5 billion in the next 20 years upgrading track, stations and signalling between the city and the lower north shore to allow more single-deck trains to cross the Harbour Bridge. It says the work could be carried out largely while trains were still running.

But the proposal echoes those being developed within Transport for NSW since at least 2008, which it has ruled out because of the disruption they would cause.

Analysis the Herald has obtained shows Transport for NSW concluded that for about ”four years there will be significant changes to the network operation in the CBD, with major disruption to operations, including no City Circle services from Central to Wynyard for three to four years (option dependent)”.

In fact, the disruption could be more intensive under the proposal by Infrastructure NSW.

The Transport for NSW proposal assumed the construction of a ”city relief line” or extra tracks between Redfern and Wynyard. These would help mitigate the impact on services while the existing tracks were overhauled and rerouted. But Infrastructure NSW proposes no spending on new CBD tracks for the next 20 years.

Switching to single-deck trains may sound simple but getting any extra capacity out of smaller trains with more doors would require rebuilding Wynyard and Town Hall station platforms.

It would also require closing lines so the complicated criss-cross of tracks between Redfern and Central could be rebuilt. Infrastructure NSW acknowledges that ”junction remodelling” would be needed to link the inner west and north shore lines south of Central.

Internal Transport for NSW documents say the work would cause a big disruption on all lines for three to four years.