Auckland's rail network could require a further $1.4 billion in upgrades - on top of the $2.5b to be spent on the City Rail Link - for it to hit maximum potential.
State-owned enterprise KiwiRail has told Auckland Council the city's existing rail network needs costly upgrades - some described as urgent four months ago.
But while the CRL gained momentum with Prime Minister John Key announcing the Government will make provisions for work to start two years earlier than planned, councillor George Wood says he's worried about a lack of progress in KiwiRail's concerns being addressed.
Wood believes those concerns strike at the heart of the CRL's efficiency and the promise Auckland will have a state-of-the-art rail network by 2023.
Minutes from a council infrastructure committee meeting last September, which councillor Wood attended, detail a KiwiRail presentation and a diagram that showed about $400m was needed prior to the CRL opening to upgrade the wider rail network.
On top of that, between $500m and $1b would be needed "post-CRL".
General manager David Gordon said those figures were initial projections.
"Until more formal costings are done, they remain KiwiRail's general view of the level of spending required in Auckland in addition to the CRL," Gordon said.
Wood said a number of issues needed to be addressed including trains being managed in Wellington instead of Auckland, freight services running on passenger routes and trains causing traffic congestion at level crossings.
"I'm really concerned we could be building this central rail system without really doing the work that's needed on the existing track network," councillor Wood told the Herald on Sunday.
"It stands to undermine the CRL and the service that will be delivered to Auckland. To get the 10-minute frequency on the Metro, you can't have freight trains impeding passenger trains.
"The bottom line is if the Auckland rail system is going to be credible, Government is going to have to put a hand into its pocket."
But the council said it didn't agree the work was needed simply to complement the CRL.
"There is nothing new in this," said Pete Clark, Auckland Transport chief strategy officer. "It was foreshadowed in the regional land transport programme last year.
"The KiwiRail presentation talks about a quantum of funding that is needed for the long-term development of the entire Auckland rail network to accommodate anticipated growth in both passenger and freight services. It is wrong to assume that this funding is needed because of the CRL.
"Complex and expensive projects such as level crossing improvements are needed regardless. Prioritising and funding rail network improvements will be part of the ongoing work on the CRL."
Recently $1.14b was spent on Auckland's rail electrification project where 57 faster commuter trains were progressively introduced across the network.
And between June 2007 and June 2010 the network's Western Line was duplicated in a project that came in at about $420m.