Under a £62m deal, Carillion will upgrade the existing track and infrastructure on the route from London to Corby over the next two and a half years.
Carillion’s 50:50 joint venture with Powerlines Group will also undertake work to complete the electrification of the route under a deal expected to generate revenue for the JV of £260m over the next three years.
Work will commence shortly on the project.
The firms were first awarded a contract back in 2014 but the electrification project was paused in the face of major cost over-runs pending a project review by Government.
Keith Cochrane, interim chief executive, said: “We are pleased to have agreed these awards with Network Rail, demonstrating that we continue to have the support of key customers and win important new contracts.
“The group faces significant challenges, but we remain focused on executing our disposals programme, taking cost out and strengthening our balance sheet.”