About 150 metres long, Wendy is gigantic by any measure.

The third tunnel boring machine for the second stage of Sydney's multibillion-dollar metro railway, Wendy will worm her way from Chatswood to the edge of Sydney Harbour, churning through thousands of cubic metres of earth to create a tunnel for driverless trains.

Wendy, one of two tunnel boring machines at Chatswood, starts tunnelling south towards Blues Point. Credit:Louise Kennerley

In several weeks, another as-yet-unnamed boring machine set up beside Wendy will also start its journey south to Blues Point, creating a twin tunnel for the mega project.

The boring machines exemplify the size and complexity of Australia's largest public transport project, which by 2024 will result in tens of thousands of passengers being carried on driverless trains every day under Sydney Harbour and the central city.