The union representing rail workers says KiwiRail is a "green-washing hypocrite" for signing up to fight climate change after scrapping electric locomotives in favour of diesel trains.

While the company says the standardised diesel fleet will get more freight off roads to rail and is exploring alternative fuel, the Rail and Maritime Transport Union says its members were ''laughing'' at KiwiRail signing up to the 60-member Climate Leaders Coalition.

The union also took a swipe at the government for not acting on a promise to ditch the deal. A spokeswoman for Transport Minister Phil Twyford today said the government would not be commenting on the issue.

The union's general secretary Wayne Butson said the climate change statement signed last week by businesses was a cynical public relations manoeuvre.

"This polygamous marriage of convenience between a gaggle of New Zealand's worst polluters may impress some people, but rail workers are not so easily fooled."

The company plans to replace its current fleet of EF class electric locomotives with DL class diesel trains imported from China, which have similar pulling power but promise 30 per cent fuel savings on current diesel locomotives.

Butson said the new diesel locomotives on currently electrified rail tracks will burn an extra eight million litres of diesel per year, spewing an additional 12,000 tons of carbon dioxide per year into the atmosphere.

In response to the union claims, KiwiRail referred to an article by its chief executive Peter Reidy who, last week, acknowledged there would be some who would say KiwiRail had ''no right to lead a discussion on climate change'' given the decision to replace electric locomotives on a section of the central North Island Main Trunk line.

''This tiny, orphan fleet is not helping us to get more freight on to rail through simplifying our operations and standardising our assets.''

He said there was also a bigger, longer-term picture that KiwiRail was working on for its busiest routes between Auckland, Hamilton and Tauranga.

''The energy for that may come from electricity, if we invest in the infrastructure required, or it may come from a whole new fuel source for rail – hydrogen.''

KiwiRail's new DL locomotives are built in China. Photo / Stephen Barker

KiwiRail says the electric trains are more than 30 years old and the 15 locos from the original fleet of 22 were nearing the end of their useful lives.

Electric trains ran only between Hamilton and Palmerston North and trains needed to be connected to diesel locomotives and back again after they had run down the central North Island.

The deal for the 20 Chinese-built locomotives was worth about $75 million.

The union also says it wants the government to live up to pre-election promises to stick with electric trains.

In the lead up to last year's election Labour had said it would instruct Kiwirail to retain an electrified network between Hamilton and Palmerston North and work on an evidence-based plan to progressively electrify other key parts of the network.

In a briefing paper to the incoming government KiwiRail said its board had been told in the lead up to the election it would get a notice to cease and desist the de-electrification programme.

By March the state-owned enterprise hadn't received any instructions.

Butson said union members we're unhappy about the lack of action from government.

"Rail workers helped this government into power with our time, money and votes." he said.