Melbourne's train tunnels are destined to remain a telecommunications dead zone for another three to six months, breaking a promise by the former Napthine government to fix the City Loop black spot by the year's end.

It is feared mobile services could interfere with the digital train radio system train drivers use to communicate with Metro's train control centre.

Sorry, you're cutting out: City Loop commuters at Flagstaff station. Credit:Craig Abraham

The long-running project to provide mobile services in the Loop is being led by government corporation VicTrack, in partnership with Optus, Telstra and Vodafone.

A VicTrack spokesman said the start of mobile phone services in the City Loop, announced earlier this year by the former government, had been delayed.