Most of the contract will see Tata Steel's Scunthorpe plant supply around 140,000 tonnes of rail a year.
Patrick Butcher, Network Rail's group finance director, said: "We are renewing and enhancing more and more of Britain's railway over the next five years and it's crucial that we have a trusted and secure supply chain to help us achieve that safely and efficiently."
The deal will account for around 5% of the annual steel output from the Scunthorpe site.
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