KiwiRail is yet to make a decision on whether it will reinvest in one of its electric locomotive fleets.

The Rail and Maritime Transport Union expected an announcement to be made today on the 17 electric locomotives operating between Palmerston North and Te Rapa.

Diesel engines take over for the rest of the main trunk line, which has never been electrified.

But the electric locomotives are worn out, and KiwiRail has been considering whether to upgrade or replace them, or to revert back to diesel entirely.

It has spent the last 15 months considering the options and is still in the process of consulting engineers and suppliers.

Rail and Maritime Transport Union general secretary Wayne Butson is worried KiwiRail will choose diesel.

"KiwiRail recently put out a notice to all its staff saying that it was going to investigate the use of electric cars, and here we are talking about the replacement of electric locomotives with dirty diesel. It just makes absolutely no sense."

The fleet's use is currently saving close to eight million litres of diesel annually, and Mr Butson said reverting to diesel is not a good idea.

"Of course they're burning fuel going uphill, and they're burning fuel going downhill, you know it just makes no sense to replace a newnewable energy source with one that's entirely reliant on dirty diesel."

KiwiRail chief executive Peter Reidy said there was no timeframe for the decision.

He said regardless of which option is chosen there are environmental gains to be made if freight moves from road to rail.