Double-decker: Waratah trains. Credit:Domino Postiglione The North West Rail Link, to run from Cudgegong Road past Rouse Hill to Epping and then Chatswood, will be Sydney's first privately-operated commuter rail line. The twin 15-kilometre tunnels for the project are being built with a six-metre internal diameter – too small to fit Sydney's existing double-deck fleet. In time this will mean much of the rest of Sydney's train system will also need to change to single-decker trains. The government plans to extend the North West Rail Link past Chatswood station through the city in another harbour crossing, linking it to the Bankstown Line and the Illawarra Line to Hurstville. But the insistence on building the North West Rail Link with smaller tunnels is controversial. The Beecroft-Cheltenham Civic Trust, for instance, is planning a public meeting for Tuesday morning to protest against the decision.

And the claim by Mr O'Farrell and Transport Minister Gladys Berejiklian that single-decker trains can accommodate more people than double-deckers is also contested. Fairfax Media has in the past reported on analysis commissioned by Transport for NSW that shows that single-decker trains do not carry more people. Mr O'Farrell prefaced his comments on Monday by saying they would be "controversial". "There are more commuters going into London than come into Sydney and they're catered in good comfort by single-deck trains," he said at the press conference at Bella Vista to announce tunnel machines would be in the ground for the North West Rail Link by October, earlier than anticipated. Graham Currie, the chairman of Public Transport at Monash University in Melbourne, said double-decker trains could carry between 20 per cent to 30 per cent more people than single-decker trains.

"I know that because we had a look at it in Melbourne and I’ve seen the numbers in Melbourne," Professor Currie said. But Professor Currie said there were advantages and disadvantages with both types. "Double-deck trains are bigger. They carry more people. But they cost more to build," he said. It was also easier to run more single-deck trains. "I don’t think it is a clear-cut decision either way: I would not myself be so definitive to say one is better than the other." The reason double-decker trains do not carry double the number of people as single-deckers is that passengers tend to bunch near the doors of the larger trains.

Introduced in mid-'60s Double-decker trains were introduced to Sydney in the mid-1960s. The silver double-decker S-Sets, which still run on Sydney's train system, were introduced in 1972. The last single-decker trains in Sydney – popularly known as "Red Rattlers" - were withdrawn in 1992. At a cost of almost $60,000, Ms Berejiklian and two department officials visited London and Paris last year. Paris, in fact, runs large numbers of double-decker trains on its "RER" lines, which run longer distances than the city's metro system. Asked about the fact that Paris is expanding its fleet of double-decker trains, Ms Berejiklian said: "In Paris they have single and double-deck trains for long distances. Some of the newest long-distance trains being built around the world are actually single-decks, so in Paris I saw both the doubles and the singles."

She said the government's plans for Sydney's train system included a mix of double-deck and single-deck trains. At the press conference on Monday, Mr O'Farrell and Ms Berejiklian said Transport for NSW would target western Sydney workers for the mammoth North West Rail Link project. The train line will include eight new stations and 4000 commuter car parking spaces, and is planned to open in 2019. The government's hand-picked chief executive of Sydney Trains, Howard Collins, has said he had warmed to Sydney's double-decker trains. "Let me be a little bit controversial here, because you've got to be a bit," Mr Collins told a business lunch last year.

"Double-deck trains – go to Paris – see how the RER pounds those trains at 24 trains an hour. The design is different, they're still double-deck, but there are solutions," he said. Mr Collins said he initially wondered what Sydney was doing with so many double-decker trains. "I've sort of become a little bit warm to them already," he said. The most recent double-decker trains ordered by the NSW government were the Waratah trains, ordered in 2006.