Melbourne Metro's oldest trains, the 1980s Comeng fleet, are to be dumped onto its most-strained lines in the city's north and north-west, leaked internal documents reveal, while other lines will get new, high-capacity trains.

The documents also show that routes in the city's fast-growing north, west and south-east are under enormous pressure, prompting Metro Trains to call for "urgent" action to upgrade lines that are at capacity during peak hour.

The fleet of 80s vintage Comeng trains are being dumped on busy lines in the city's north. Credit:AAP

An internal company document obtained by The Age warns that Melbourne's rail commuters cannot wait until the Metro Tunnel is built in 2025, with the rail operator advising that several lines need a solution now.

The northern group – which includes the Sunbury, Craigieburn and Upfield lines – is already at capacity during peak hour, meaning no more services can run through the City Loop.