The £220m contract will include design and construction of the new line.
It is thought 500 jobs will be created by the project between now and 2015.
The Borders Railway will open for service in 2015 and will provide a link from Edinburgh through Midlothian to Tweedbank.
30 miles of railway will be provided by the project and will reach three existing stations and seven new ones: Shawfair, Eskbank, Newtongrange, Gorebridge, Stow, Galashiels and Tweedbank.
David Simpson, Network Rail route managing director for Scotland, said: "BAM Nuttall has already been working closely with Network Rail in drawing up initial designs for the new line and can now carry that momentum into the delivery of the main construction phase as we work to complete the new line for the start of services in 2015."
-- Advertisement --
Stephen Fox, Chief Executive of BAM Nuttall, added: "This scheme brings together the engineering strengths of the BAM Group and will create significant local employment opportunities. In addition it will provide a much needed rail link to those rural communities living in the Scottish borders region."
Scottish Transport Minister Keith Brown added: "Work is already well underway on the site and the people of the Borders and Midlothian areas will soon be able to see the actual construction begin and the Borders Railway begin to take shape. And all the partners will continue to work together to see if we can find a way to deliver the project even earlier than 2015.
"This is a very exciting milestone for a community which has waited over 40 years for the return of the railway and they will soon begin to reap the very real benefits of this project in terms of investment to the area, increased tourism and improved accessibility to employment opportunities."
Construction is due to begin early 2013 and expected to be finished by summer 2015. The capital construction cost of the line will total £294m.
(IT)
