Both fleets would run on electricity from overhead power wires switching to alternative energy sources north of Waikanae and Upper Hutt.
The upgrade would provide capacity for supplementing peak demand on both the Hutt Valley and Kāpiti Lines which alone provide transport for 84 per cent of rail customers.
Kāpiti's regional council representative Penny Gaylor was excited about the business case.
"We've had a number of business cases that have been done over the last six or seven years but, of all the ones I have seen, this is the most robust one and it also provides the best benefit cost ratio which is the real game changer."
The conservative benefit cost ratio range was 1.3 to 2.6.
"For the first time it provides a compelling argument to Government."
She said the replacements would also be able to support the network though the day helping futureproof capacity issues, which in turn could see stops such as Ōtaki get more rail movements.
And the business case was in the "right phasing for the budget process that the Government goes through for next year".
Sustainable transport committee chairwoman Barbara Donaldson said the biggest issue facing the region's rail network was providing sufficient capacity to meet future demand.
"Our rail network reached a new annual patronage high of 14.3 million passengers in June 2019, representing an increase of 5.7 per cent year on year.
"The peak patronage rates were even higher, rising 7.3 per cent year-to-date, with our busiest lines being Hutt Valley and Kāpiti.
"Trains running longer distance services between Wellington and Masterton and Wellington and Palmerston North require refurbishments or replacements and patronage growth on these services has already exceeded the available capacity for both fleets.
"The increase in train-users both within and beyond the metro network shows an urgent need to plan and fund a fleet solution that avoids capacity shortages across the whole network as early as the mid-2020s.
"These modern rail services will improve customer experience and allow an increase in service frequency on the Wairarapa and Manawatū corridors.
"Greater Wellington is keen to work with stakeholders to map out a solution to fund and deliver these essential services for the future."
Kāpiti mayor K Gurunathan said the rapidly growing patronage of the Kāpiti commuter rail system was expected to hit capacity problems as early as the mid-2020s but urgent action by GWRC to deliver additional new trains would not happen until 2025.
He said it was unclear how many of the 15 units would be used on the Kāpiti line.
"At present there is only one Capital Connection return trip between Palmerston North and Wellington with stops at Ōtaki, Waikanae, Paraparaumu and Paekakariki.
"The trains are old.
"GW's Long Term Plan (2018 — 2028) had included an intention to have two return trips and the purchase of 17 EDMU trains.
"But the business plan for this project has been kept under wraps.
"The LTP did not include the potential of the EDMUs being used as a shuttle between Waikanae, Ōtaki and Levin.
"GW had delayed giving details of its business plan despite my OIA request.
"In early July, I wrote to Transport Minister Phil Twyford reminding him that Kāpiti's total support for the $6.4 billion Lets Get Wellington Moving transport package depended partly on investment in public rail transport to Ōtaki and beyond.
"I urged for an earlier investment than projected by GW's LTP to reduce parking problems at the Waikanae rail station, encourage rail public transport and reduce traffic congestion at Ngauranga Gorge and the Terrace Tunnel.
"And the need to change public behaviour from private vehicles towards public transport.
"In his July 29 response the minister said NZTA, GW, KiwiRail and Horizons were working on a short term solution to keep the Capital Connection in service until a long-term solution.
"On the later, he added, two processes were being led by GW: 'The first is the development of a regional rail plan to guide further investment in rail within the Wellington region (and exploring inter-regional connection). The second is the development of a business case for longer-distance rolling stock for the Wairarapa and for connections through Kāpiti, Horowhenua to Palmerston North.'
"The minister said that both draft business cases were 'intended' to be reviewed by NZTA and 'other stakeholders' in September.
"The business cases, he added, will explore the potential for increased frequency of services as well as detail the lead-in times for bringing any new units into service.
"Further engagement with KCDC was promised."
