A significant number of Carillion's Network Rail (NR) contracts have been acquired by Amey Rail Limited, safeguarding around 700 jobs.
NR said the sale provides reassurance to the rail industry and continuity for projects around the country.
The transfer of Network Rail contracts operated by Carillion Construction Limited includes projects in the East Midlands, London and north west England.
Matthew Steele, commercial director at Network Rail, said: "This is a positive step for the industry, ensuring the delivery of a number of major rail projects across the country whilst safeguarding an expected 700 jobs and the wider supply chain.
"We look forward to working with Amey to enable a smooth transition and ensure the ongoing safe working on our sites.
"We do recognise that this has been a very unsettling period for the employees of Carillion and would like to thank them for the continued commitment to the delivery of these projects.
"We remain focused on the transfer of remaining projects and employees to new arrangements over the coming weeks and months."
Transport Salaried Staffs' Association leader Manuel Cortes said: "I am pleased to hear that our members' jobs will be protected by Amey's purchase of part of the Carillion rail business and that vital infrastructure work will continue.
"But the collapse of Carillion shouldn't be an excuse to undermine our members' contracts. I am urgently seeking a meeting with the company to seek assurances that our members' terms and conditions including pensions will be safeguarded."
Rail, Maritime and Transport union leader Mick Cash said: "RMT's preferred option since the collapse of Carillion has been for their rail works to be taken directly in house but we recognise that today's announcement should guarantee up to 700 rail workers' jobs and we will now be sitting down with Amey to work through the key details around continuity of employment and pensions."