The £10 million deal is part of an overall £57m Scottish Government-funded investment in the line linking Inverness and the Central Belt.
The contract will involve extending platforms 1 and 2 at Pitlochry station and carrying out modifications to extend the track layout.
Infrastructure works will also be carried out at Aviemore station – extending the loop (section of double-track) to the north of the station.
In addition, signalling systems at Pitlochry and Aviemore will also be altered and upgraded as part of the overall project.
These works will enable ScotRail’s longer InterCity trains (HSTs), which will start to be introduced on the route later this year, to pass each other more efficiently at both stations.
The work will help deliver an hourly service, with average journey times reduced by around 10 minutes. It will also enable potential improvements to the times of the first and last trains between Inverness and the Central Belt and to provide a better distribution of calls at the intermediate stations between Perth and Inverness.
This is the second phase of investment in the Highland mainline. In 2012, the initial phase of the project increased the number of services from nine to 11 trains per day in each direction.
Humza Yousaf, minister for transport and the islands, said: “The awarding of the Highland mainline works contact is an important milestone in the progress of the project, which is on track to complete in little over a year.
“The works planned to take place at Aviemore and Pitlochry allow for far more efficient crossing of trains, which along with the soon-to-be-introduced HSTs and infrastructure enhancements, will deliver faster more frequent journeys between Inverness and the Central Belt.”
Matthew Spence, route delivery director for Network Rail, said: “This work is vital to the introduction of InterCity trains on the line and the improved timetable that will deliver more services, improve stopping patterns and provide better connectivity for customers.
“Throughout this project we will work closely with our contractors, train operators and other partners to deliver in the most efficient and cost-effective way possible while minimising disruption for both lineside communities and passengers.”
Jerry Dickson, regional director Bam Nuttall, added: “We are delighted to have been awarded this contract and look forward to delivering this important enhancement to the Highland mainline.
“Our team will be looking for opportunities to work with the local community to identify opportunities to leave a positive lasting legacy from the work at both stations.”