The O'Farrell government was told it would save $200 million from building the north-west rail link with smaller tunnels - a decision that will mean the new line will never be able to run double-deck trains with more seats.
Documents obtained after a freedom of information battle also reveal the lengthy effort inside parts of the transport bureaucracy to warn the government about the major inconvenience to northern suburbs commuters from terminating the $8.3 billion north-west rail link at Chatswood.
And they show that transport planners inside Transport for NSW are working on the basis that the next major rail project after another harbour rail crossing is built will be a train line to Sydney's northern beaches.
Fairfax Media has obtained multiple internal reports and emails into the decision to build the north-west rail link as a privately operated shuttle to Chatswood station.
The Herald has previously reported on leaked analysis showing that almost half the commuters leaving the north-west rail link trains at Chatswood will be unable to get on the next service to the city, because those north-shore line trains will be too crowded.
But the documents obtained using Government Information (Public Access) laws provide a fuller account of the safety and cost concerns in parts of the bureaucracy about the plans for the north-west rail link.
"It is not yet clear whether these passenger loads are within the practical limit of platform capacity," wrote a division director within Transport for NSW, Michael Magney, to the head of the north-west rail link project, Rodd Staples, on May 16, 2012, about a best-case scenario at Chatswood Station.
Mr Staples' response is not included in the documents.
But a month later, Premier Barry O'Farrell and Transport Minister Gladys Berejiklian announced the north-west rail link would be built as a single-deck shuttle to Chatswood, breaking a promise they would run trains on the new line all the way to the city.
Ms Berejiklian said that commuters changing at Chatswood for services to the city would not have a problem, because the government would be able to run as many as 24 trains an hour from Chatswood to the city by the time the north-west rail link opened in about 2019.
But multiple Transport for NSW documents show scepticism about this claim.
"This [24 trains an hour] would not be possible within the CBD without very significant investment in station and systems," Mr Magney wrote in another email to senior staff, including Carolyn McNally, the head of planning in Transport for NSW.
The decision to build the north-west rail link with smaller tunnels that will never fit double-deck trains has alarmed transport experts and commuters.
Fairfax Media has previously reported that Transport for NSW commissioned analysis showing double-deck trains could carry more people than single-deck trains, even though it continues to insist single-deck trains can carry more.
Another Transport for NSW document from May 2012 says the decision to build small tunnels offers "the opportunity for some direct project cost savings through a reduced tunnel diameter and station size," putting the savings at $200 million.
But the government has since said it could spend even more than that converting the existing Epping to Chatswood line to run double-deck trains.
Despite the analysis showing thousands of commuters at Chatswood will not be able to fit on trains to the city, Transport for NSW and Ms Berejiklian insist the station will work well.
"The modelling work being undertaken right now on the Chatswood interchange shows that customers will seamlessly interchange on a platform which will perform better than other locations on the current network, while the second harbour crossing is being constructed," Ms Berejiklian said.