Sydney commuters travelling on the new Metro Northwest line will have to become more used to standing when it opens in early 2019.

A notable feature of a full-scale carriage unveiled on Thursday is the absence of the seats that commuters have become accustomed to over the decades on the city's double-decker railway carriages.

Passengers on the new trains will have to become more accustomed to standing, especially during peak travel periods.

However, Transport Minister Andrew Constance was unapologetic about the fewer seats, emphasising that "ultimately this is the very nature of metro-style trains".

"This is what metro systems do. This is a turn up and go, high frequency service – there is a train every four minutes," he said.