Carved out of sandstone about 30 metres below North Sydney's streets, the giant underground cavern for a new train station will be one of the deepest on Sydney's new metro rail line.

Yet Victoria Cross station could have ended up much deeper – up to 60 metres below the heart of North Sydney, under engineers' early concept designs.

It would have allowed it to join the ranks of some of the deepest underground stations in the world, but would have meant that lifts were the only option to move thousands of commuters every hour between its 168-metre long platforms and the busy streets above.

"You start to get to a point where escalators are no longer working. We were determined not to get to that point because it starts to feel more inconvenient and it's harder to deal with larger volumes," Transport for NSW secretary Rodd Staples said on Monday.