Industrial unrest has struck Melbourne's high-tech next-generation trains two years before the first of them is due to hit the tracks.

The union that represents Metro's train drivers has instructed its members not to co-operate with the rail operator in its preparations for the arrival of the 65 trains to Melbourne's network from mid-2019.

The high-capacity trains will relieve peak-hour overcrowding on two of Melbourne's busiest lines, in part by prioritising standing room over seating space in their internal configuration.

They will be built to carry between 1200 and 2000 passengers each, depending on their configuration.