KiwiRail will continue to spend $2m each year to maintain the overhead electric wires between Hamilton and Palmerston North, even though they've scrapped the only trains that use them.
The state-owned company is replacing the 30-year-old EF electric fleet with diesels because they are cheaper, more readily available and more versatile.
But the overhead cables that power the EF fleet will remain in case KiwiRail or a future government decides to reinstate electric engines on the line.
They will also remain electrified to prevent the theft of expensive components and conductors.
"The cost of maintaining the electric infrastructure is about $2 million a year," KiwiRail said in a statement.
"KiwiRail is continuing to spend this money to keep the use of electric trains as a future option should the situation change."
The company is also defending its decision to move from clean energy to diesel power, which will see its carbon footprint increase by 10 percent.
"It will increase KiwiRail's emissions by 14 thousandths of 1 percent of New Zealand's Greenhouse Gas emissions," a spokesperson says.
"Emissions from rail are less than 1 percent of all transport emissions, with transport accounting for 17 percent of New Zealand's overall emissions."
KiwiRail did consider electrifying the entire North Island rail network, but concluded the $4.5 billion cost would be better spent elsewhere.