THE New South Wales Government this week announced the multi-billion dollar tender process to build the North West Rail Link has started, while also revealing that the new line will be Sydney's first to feature new generation single-deck, high frequency train services.

The announcement was made on Wednesday by Premier and Minister for Western Sydney Barry O'Farrell and Transport Minister for Transport Gladys Berejiklian.

The Premier said Expressions of Interest will be called later this month for the construction contract to build 15.5 kilometres of tunnels between Bella Vista and Epping, adding that the first tunnel boring machines are on schedule to be in the ground in 2014.

As part of Sydney's Rail Future - a plan to transform Sydney's rail network that was also announced this week - the first single-deck trains will operate from the North West to Chatswood, providing customers with at least 12 trains an hour in the peak, or one every five minutes.

Sydney's Rail Future also outlines plans for a second rail crossing under Sydney Harbour that will allow for direct trains from the North West to the Sydney CBD via Chatswood and beyond.

There will be three major contracts for the construction of the North West Rail Link project: Tunnelling; Surface construction works; and Rail systems, trains and operations.

"We are now beginning the tender process for the multi-billion dollar construction contract to build the longest and deepest underground rail tunnels in Australia," Ms Berejiklian said.

"There will be a major industry briefing next week to outline the contract structure, allowing the NSW Government to get on with the job of delivering this vital new infrastructure for the people of the North West."

According to the Minister, early construction work on the North West Rail Link is due to commence later this year.

In the 2012-13 State Budget released last week, the NSW Coalition Government allocated $360 million towards the North West Rail Project and expects to invest $3.3 billion over the next four years.