Passenger David Coates discusses the Wairarapa line's woes as he catches the 4.25 service over the hill on Wednesday.

A $300 million fleet of modern, diesel-electric trains is being sought for the troubled Wairarapa commuter line, but they probably won't be operational until at least 2023.

Greater Wellington Regional Council is putting together a business case to obtain the 15 new electro-diesel multiple units (EMDUs), which it says will dramatically improve service on the line.

The trains would be able to run on electricity along the electrified parts of the Wellington region's network and on diesel outside of it. They would also be more compatible with other trains on the network.

ILLYA MCLELLAN/STUFF Greater Wellington Regional Council is hoping to replace ageing trains on the Wairarapa line with modern diesel-electric trains. (File photo).

The line that carries commuter trains to and from Wairarapa is not electrified between the Hutt Valley and Masterton.

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Any funding application would not be made until at least the 2019-20 financial year, with the first train likely to be delivered about four years after that.

GRANT JACKSON/SUPPLIED Commuters walk along the tracks toward Maymorn Station after a train breakdown on the Wairarapa line in December.

It was yet to be determined who would pay for the new trains, or how the cost might be split up between central and local government.

A report that will be discussed by the council's sustainable transport committee on Wednesday says these "versatile, longer distance trains" would provide enhanced levels of service and capacity on existing routes, and "invigorate" regional development opportunities across the lower North Island.

"As a minimum, the new fleet will allow additional services from Masterton and Palmerston North, and provide additional capacity and flexibility on the electrified metro networks."

ILLYA MCLELLAN/STUFF The new trains would be more compatible with others on the network, meaning finding replacements for trains out of service would be easier. (File photo).

The plan for the new, four-carriage trains is part of the council's Long-Term Plan for the line, which also includes upgrading the entire track and adding an extra track between Trentham and Upper Hutt.

It follows two funding bids by the council to the Government last year to renew ailing track infrastructure on the line and improve reliability and resilience at a cost of nearly $200m.

It is hoped that funding will be approved in this year's Budget.

JACK BARLOW/STUFF Punctuality on the diesel Wairarapa line improved to almost 69 per cent in December, well below the 90 per cent average on electric lines (file photo).

A council spokesman said there was no point purchasing the new trains until the line upgrade had been confirmed.

It typically took about four years from the time an expression of interest was issued for the first train to be rolled out. The delivery of the remaining trains would take even more time, he said.

"That time is needed because each train system needs to be designed and built for the network they are being run on ... and each train needs to be tested, and staff trained how to operate and maintain them."

CALEB HARRIS/STUFF Masterton Mayor Lyn Patterson says there will be more pain before there is some gain for Wairarapa rail commuters.

The Wairarapa line has been dogged by lateness in recent years, mainly due to its ageing tracks, which mean trains are subject to speed restrictions. Customers have also complained recently about soaring temperatures on board.

An independent investigation was launched in December following more than a dozen mechanical faults in six months. Draft reports from that investigation are due back late next month.

But despite the issues, the number of passengers using these trains in peak times has actually increased by 5.3 per cent in the year to December.

The council report acknowledged that statistic, as well as a 10 per cent customer satisfaction increase in its latest survey, was probably "incongruous".

"These statistics provide all the more reason why, despite the challenges, our customers deserve a better service and a number of initiatives are underway to improve as many aspects as possible," it said.

Masterton Mayor Lyn Patterson said while she supported bringing in the new trains, upgrading the rail line remained the top priority.

"If we can't bring our rail tracks up to speed by investment, then we can have the latest carriages in the country, it may help with over-crowding, but it won't address the fundamental problem, which is the neglect of our rail line from Wellington to Wairarapa."

Patterson commuted for six years and understood commuters' frustration, but said there had been "years and years of neglect" of Wairarapa's rail infrastructure and there would be no immediate fix.

"We're going to have to put up with a little more pain before we can get some gain. Unfortunately, that's the reailty."

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