The labyrinth of tunnels beneath the heart of Sydney's skyline offers the first glimpse of the metro train station carved out of sandstone over the past nine months.

Within four years, thousands of commuters will pile on and off metro trains every hour at the new Pitt Street station, fewer than 100 metres from Town Hall station in the central city.

Tunnels for the new Pitt Street station, which is part of the second stage of Sydney's $20 billion metro rail line. Credit:Wolter Peeters

On Friday, the first boring machine to snake its way from Waterloo, in the city's south, reached what will become one of two 160-metre platforms for the Pitt Street station, which is part of the second stage of Sydney's $20 billion metro rail line.

Named Nancy, it is one of five giant boring machines churning away to form twin 15.5-kilometre rail tunnels stretching from Chatswood in the north, under Sydney Harbour to Pitt Street station and three others in the CBD, and onto Sydenham in the south.