KiwiRail is installing up to 160,000 concrete rail sleepers a year on its network as it phases out the wood equivalents.
About four million wooden sleepers need to be replaced. The rail company says the replacement programme was ramped up in the mid-2000s following years of under-investment.
KiwiRail network performance general manager David Gordon says concrete sleepers have a much longer life span than their wooden equivalents, lasting about 80 years.
He says KiwiRail is concentrating efforts on key freight routes, including Auckland to Tauranga.
The sleepers were orginally imported from China, until a 10-year $100 million contract was signed with manufacturer Humes.
Mr Gordon says all the concrete sleepers are now being made in New Zealand.