The single rail track through the Whangamarino section of the Hamilton to Auckland rail network can be seen on the left, with Meremere and the Waikato River on the right.

Earth dug from under Auckland city could be spread out across a Waikato swamp to double-track a notoriously slow section of main trunk line.

Just one track runs through the Whangamarino Wetland - home to rare species of fish and plants - and at 15 kilometres long and built on soft soil, it makes for slow travelling for trains using the route.

But a new plan being touted by KiwiRail would involve sand, rock and soil extracted from the Auckland City Rail Link dumped in the North Waikato as the foundation for a second track.

DEREK FLYNN/STUFF KiwiRail are in the very early stages of the investigation says group general manager of investment, planning and risk David Gordon.

The Waikato Regional Transport Committee moved to support the concept in principle after KiwiRail made a presentation on Monday.

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"They are saying, if we wanted to increase the ability of the [Hamilton to Auckland] rail link for both passengers and freight at some stage, we need to double-track that 15km," committee chairman Hugh Vercoe said.

CHRISTEL YARDLEY/STUFF KiwiRail are investigating using tunnel spoil for other rail projects outside of Auckland.

Two million tonnes of spoil will be extracted from Auckland's CBD from 2020 when tunnelling operations begin, Vercoe said. Each day, 450 trucks will be needed to cart the spoil away.

The Whangamarino double-track option would allow KiwiRail to limit its effect on Auckland city congestion by sending trucks a short jump to the Ports of Auckland, where the spoil would be transferred to rail and carted south.

"It would be available to widen the existing track through the Whangamarino swamp. That would allow them to double-track that area," Vercoe said.

CHRISTINE CORNEGE/STUFF Hugh Vercoe, the chairman of the Waikato Regional Transport Committee, has given the proposal a tentative thumbs-up.

The double tracking would serve Waikato Tainui's Ruakura project too, but Waikato Tainui chief executive Donna Flavell is yet to be informed of the detail's of KiwiRail's plan.

"Waikato Tainui is unaware of the project detail, however we expect to be fully consulted, consistent with the relevant Resource Management Act provisions," Flavell said.

"In principle, we support improved rail mobility between Hamilton and Auckland. However, it must be undertaken in a manner that protects the health and wellbeing of the Whangamarino Wetland and with minimal impact on the surrounding environment including our tupuna awa."

SUPPLIED/CRL Two million tonnes of spoil will be excavated when the Auckland City Rail Loop tunnelling work starts.

At more than 7000 hectares, the wetland is one of the largest in the country hosting rare fish, bird and plant species.

Environment groups were contacted, but none would not comment on the effects of double-tracking until they had detailed information to hand.

KiwiRail's investment, planning and risk general manager David Gordon said other rail projects could be in-line for Auckland's spoil but the planning, and that of the Whangamarino double-tracking, was in the very early stages of feasibility testing.

MARK TAYLOR/STUFF Waikato Tainui chief executive officer Donna Flavell is yet to have the full details of KiwiRail's investigation.

"While it is currently more an idea than a proposal, KiwiRail believes it is an option worth investigating, and is starting to discuss it with other interested parties," Gordon said.

He said the Whangamarino Wetland was environmentally significant and any work that took place would have to be designed with the ecology of the area in mind.

"The Waikato Regional Transport Committee is the first party KiwiRail had spoken to about this idea and there was a clear need to talk to others, in particular Tainui, to understand their position."

​The Whangamarino section was the only remaining single-track section between Auckland and Hamilton, apart from the rail bridge at Ngāruawāhia which is only 350m and does not cause the same delay the 15km stretch does.

Rail freight and passenger services are growing in the economic golden triangle of Auckland, Hamilton and Tauranga and a second track will need to be added.

Hamilton City councillor Dave Macpherson, who is working on the interim commuter rail service between Hamilton and Auckland, said KiwiRail's idea would have little bearing on current work but it would make a big difference in the long term.

"That's where it will help the most because it will give you more potential slots because you can have two lines running. But it will also be built to a higher standard than the ones that were built in the 1910s or earlier," Macpherson said.

"The overall efficiency of the thing improves remarkably when you can go both ways at the same time."

Double-tracking feeds into Transport Minister Phil Twyford's transport corridor plans between the cities, he said. All relevant agencies - councils, KiwiRail, Auckland Transport, MBIE and NZTA - have been called to a second meeting in Twyford's office on June 25.

"The start-up rail is listed specifically on the agenda," Macpherson said.