After early hiccups, construction on the four-kilometre rail bridge that will form part of the Sydney Metro Northwest project is rattling along, the project director says.

Throughout Sydney's north-west, from Epping to Rouse Hill, evidence of Australia's biggest public transport project is everywhere.

The twin 15-kilometre tunnels that run for just over half the route of the new line are about half dug. And the early sections of the "Skytrain" rail bridge to run between Kellyville and Rouse Hill are rising from the ground.

"We are now working on every construction site across the whole project," said Rodd Staples, the project director of both the Sydney Metro Northwest rail line and its planned extension south of Sydney Harbour.