Tunnel finished for Sydney north west rail

The fourth and final tunnel for Sydney's northwest train link has been completed.

A boring machine has cracked a tunnel at Cherrybrook Station on Thursday, making way for the rail line between Rouse Hill and Epping.

NSW Premier Mike Baird says work on the train line, which will include eight new stations, has been completed ahead of schedule.

"(It's) a great opportunity for people to improve the quality of their lives in the northwest," he told reporters at Cherrybrook Station, which is due to open in the first half of 2019.

The 36km line will run trains every four minutes during peak hour, or 15 trains an hour.