"Without this replacement programme commuters would have faced increasing disruptions to the service as the wooden poles reach the end of their lives.
"The contract is part of a $98.4 million four-year project begun last year to replace the overhead power system between Wellington Station, Melling and Upper Hutt, as well as Ngaio and Johnsonville. The funding came from central Government.
The project, which will see 1274 existing poles and 38km of overhead wires replaced, is expected to be completed in 2021.
KiwiRail and Greater Wellington Regional Council are working together on the project.
The council's sustainable transport committee chairwoman Barbara Donaldson welcomed the signing, saying it was "a major step forward in improving our rail network infrastructure and ensuring a more reliable and sustainable service to people from the Hutt Valley who make five million journeys each year on the Hutt Line".
"Significant investment is needed on the infrastructure due to the ageing nature of the network and we are hoping for further investment to upgrade the rest of the infrastructure to unlock capacity and provide a modern, reliable service for all commuters in the Greater Wellington region," she said.