The metro rail line to Sydney's north-west is one step closer to completion with the unveiling of a "mini-Anzac Bridge" over Windsor Road at Rouse Hill.

NSW Premier Gladys Berejiklian inspected the 270-metre long bridge on Monday, describing it as a "critical milestone" in the delivery of the $8.3 billion railway, Australia's biggest public transport project.

NSW Transport Minister Andrew Constance said trains would run every four minutes in each direction during peak times, "bringing reliable metro rail to this region … for the first time".

The deck of the bridge is made from 88 concrete precast segments, each weighing between 70 and 140 tonnes.