If the first two days are any guide, Sydney's travelling public has spoken – they want more frequent and reliable train services.

A staggering 140,000 people travelled on a new 36-kilometre metro line between Chatswood and Rouse Hill in the city's north west on the opening day on Sunday, forcing the private operator to quickly put on more trains to clear crowds.

A day later, 21,000 commuters chose to hop on driverless trains on the $7.3 billion Metro Northwest line in the first five hours, significantly higher than the 15,000 to 17,000 forecast.

Passengers on a metro train on Monday morning. Credit:Peter Rae

It left wide smiles on the faces of Premier Gladys Berejiklian and her closest transport advisers such as Rodd Staples, the NSW Transport Secretary and architect of the metro project.