Sydney's North West Rail Link underway

Work on Sydney's $8.3 billion North West Rail Link is running ahead of schedule with massive boring machines set to start drilling by the end of October.

NSW Premier Barry O'Farrell says a major recruitment drive is underway to hire 900 workers to build the link.

"(This) is great news for people in Sydney's northwest, who have been waiting far too long for this project," he told reporters at the Bella Vista construction site on Monday.

Boring machines, which are twice the length of an A380 airliner, will start digging in seven months, several months earlier than expected, and will eventually reach Cherrybrook in two years.

Eight new train stations will be built at Cherrybrook, Castle Hill, Showground, Norwest, Bella Vista, Kellyville, Rouse Hill and Cudgegong Road, along with 4000 commuter car parking spaces.

A train will run down the line every five minutes in peak hour and high-frequency single deck trains will operate between Cudgegong Road and Chatswood.

Mr O'Farrell defended choosing single deck trains for the line, saying the model is increasingly being used across major cities such as Paris and London.

Single deckers can carry more people, travel faster and disembark people quicker, he said.

The project is expected to be completed by 2019.