Millions will be spent upgrading the Upfield train line, but Melbourne's most sluggish peak-hour service may not improve for another seven years, when the Metro Tunnel is built.

Major upgrades promised for the line, which runs through the battleground state seat of Brunswick before reaching the safer Labor seats of the northern suburbs, will not boost the frequency of peak-hour services.

Skyrail will likely stretch over four level crossings in Brunswick and Coburg, under the Andrews government’s $6.6 billion level crossing removal plan.

But Upfield line commuters want to know why Labor won't duplicate a single section of the rail track that is causing bottlenecks on the line, as it removes the level crossings.