"There are places like the track between Clifton Hill and Jolimont that are going to be overcrowded really quickly - as in, the next three to four years - and we won't be able to run any more trains on those tracks," Mr Dobbs told The Sunday Age.

"There are also other bits of the system like the Werribee line, the northern group of services, or the Dandenong line itself, where such is the growth that it's generating significant peak-hour pressures. The tunnel is not the only way of dealing with that – one of the things that we do need to move to is high-capacity signalling."

High-capacity signalling, which tells train drivers when the line ahead is clear by putting the signals inside their carriage instead of alongside the railway tracks, would allow trains to be run more efficiently across the network. But while the technology is used in dozens of cities around the world, it has not yet been introduced in Australia.

Mr Dobbs, who ran the Public Transport Corporation in the 1990s, was the inaugural chief executive of Public Transport Victoria under the Baillieu government and sat on the original steering committee for the Metro Rail project, said high-capacity signalling was a "big technical jump - like going from a Tiger Moth to an Airbus 380". But he said it would give the state the ability to run "more trains, more reliably ... closer together".

In Victoria, the government is yet to commit to such a technology boost, although it is likely to form part of any plans to adopt the Coalition's Pakenham-Dandenong Rail Capacity Project, which Labor is considering.