The railway union is accusing the government of breaking a pre-election promise to stop the removal of electric trains from the North Island's main trunk line.
Radio New Zealand reports that KiwiRail freight trains use diesel locomotives in Wellington, switch to electric units at Palmerston North, then back to diesel in Hamilton.
KiwiRail decided two years ago to replace its electric engines with diesel.
The Labour Party told the rail operator in August it would stop that plan, so the rest of the line could still be electrified.
Transport Minister Phil Twyford said he was still talking with the company, but it was clear it could not afford to buy new electric engines.
Rail and Maritime Transport Union general secretary Wayne Butson said the electric engines were not at the end of their lives.
He said KiwiRail has not maintained them and all they needed was new wiring and electronics.
Minister for Workplace Relations Iain Lees-Galloway is to meet with staff from the electric depot in Palmerston North next week to talk about their futures.