Running single-deck trains through Sydney would not allow more people to travel on the train system, says a report commissioned by Transport for NSW and kept secret by the department, contradicting repeated claims by Transport Minister Gladys Berejiklian.

The report was being prepared at the time Ms Berejiklian and her department were drawing up plans to run smaller single-deck trains on the new north-west rail link and on other lines as well.

Roomy: A Waratah double-decker train. A single-deck model would fit 25 per cent fewer people. Credit:Simon Alekna

When Ms Berejiklian and Premier Barry O'Farrell announced in June 2012 they would build the north-west rail link as a privately-run shuttle between Rouse Hill and Chatswood, they also said parts of the Illawarra Line and Bankstown Line as far as Hurstville and Cabramatta would eventually be converted to run single-deck trains once a second rail crossing of the harbour was built.