Work starts soon on a four-year $10 million project to repair and upgrade the Bay's railway lifeline to the rest of New Zealand, the Kaimai Tunnel.
It will be the first major reconstruction of the tunnel since it opened to a fanfare of publicity 36 years ago.
Contractors will enter the tunnel on September 13 to identify what was required to future-proof it, including the key job of strengthening the floor.
It will be a one-off 36-hour opportunity while a bridge on the Waikato side of the line was replaced.
Work would include using ground-penetrating radar to review the condition of the tunnel and installing drain and water flow monitoring systems.
Details of the work needed to cope with increasing volumes of freight were outlined to a recent meeting of the region's transport committee by KiwiRail's senior projects manager Jeanine Benson.
The deteriorating tunnel floor meant the concrete slab under the rails was operating at its maximum 18-tonne axle limit, requiring speed restrictions.
"The long-term integrity of the tunnel floor is a challenge," she said.
Replacing the footing of the slab and repairing the concrete will cost $2.2 million, enabling trains to carry heavier loads faster through the tunnel.
The condition of the tunnel was assessed last year by KiwiRail, with improvements to include installing a ventilation system. Bringing the tunnel up to modern standards would also require a complete overhaul of the communications system to fibre optics to ensure safety for trains and people working in the tunnel. New power supplies will be laid to each end of the tunnel to feed lighting and ventilation systems.
A big challenge facing project planners was how work would take place safely inside the tunnel without impacting on trains. Fixing the problematic tunnel floor would see improvements to the centre drain.
The upgrade of the floor slab would take the railway to a maximum 25-tonne capacity.
• Length 8.8m emerging near Aongatete Lodge, Apata
• Built 1969 to 1978
• Bypassed the old East Coast Main Trunk Railway route through Karangahake Gorge
Other Bay Investments by KiwiRail
• February 2014: New railway sidings at Tauranga
• April 2014: Increased train sizes Kawerau to Mount Maunganui
• April 2014: Additional log train Murupara to Mount Maunganui