A four-kilometre section of single track railway in a sparsely populated part of Melbourne is causing transport problems all over the north-western suburbs, Metro says.

The Upfield railway line, one of Melbourne's quietest, goes from two tracks to one at the end of the line between Gowrie and Upfield stations.

The single track section can only accommodate one train at a time, creating a bottleneck that has long condemned Upfield line passengers to 20-minute waits between trains, even as several other lines in Melbourne move towards 10-minute frequency.

Metro has called for the single-track section of the Upfield line to be duplicated by 2017, arguing that services on the line should be increased "towards a 10-minute frequency".

In a proposal to the state government, Melbourne's rail operator says the single track should be duplicated because it also puts pressure on the Craigieburn and Sunbury lines, two of Melbourne's busiest and most prone to peak-hour overcrowding, because all three lines are linked in a northern group.