"That is more for places with the population density of England than here in New Zealand. The population of the Midlands for example is over 10 million people as against the 730,000 in Waikato/Bay of Plenty and the cost of rapid rail there is in the billions of pounds."
He said the existing rail lines were the busiest freight lines in the country and there was no capacity for a passenger commuter service as well without double-tracking large sections.
"Labour doesn't have any plan to invest for that. Labour would kick economy-fuelling freight off this important line and replace it with empty commuter carriages."
Labour's plan would invest an initial $20 million for the first stage - a passenger service of 3.5 hours once a day from Tauranga and about two hours from Hamilton four times a day.
If demand justified it, Labour would look at upgrading stations and crossings to allow faster speeds, and eventually opening up to other towns such as Rotorua.
Ardern also pledged to double the amount in a fund for regional transport projects from a maximum of $140m to $280m.
She also plans to hold a roading summit with councils within the first 100 days of a Labour Government to accelerate projects important to regions - with a focus on fixing crash black spots.