Carillion has secured a £120m contract from Network Rail, through the Multi Asset Framework Arrangement to upgrade depots and stabling sites as part of the Thameslink Programme.
Under the programme, Carillion will carry out improvements to allow the introduction of Thameslinks' new 12-car fleet.
Carillion chief executive Richard Howson said: "We will continue to work closely with Network Rail and the train operators to maximise value for money in delivering this important new phase of the Thameslink project."
The latest project will be part of the £6bn Thameslink Programme to offer more trains and better journeys for passengers on the Thameslink route running north-south through London.
After completion the Thameslink upgrade will improve train travel in London's north-south main line route, delivering more trains, seats and journey choices as well as new and upgraded stations.
In a bid to boost capacity, 12-car trains are planned to be introduced on the main Bedford to Brighton route as well as a further increase in frequency up to 16 trains per hour.
As part of the programme the new station at Blackfriars and the expanded Farringdon station will also be completed.
By 2015 the London Bridge station will be redeveloped to remove the bottleneck on the UK rail network and allow 24 trains to run per hour through central London.
The project includes the complete remodelling of the track and signalling systems which will begin in 2013.
Image: The £6bn Thameslink Programme involves the new station at Blackfriars and an expanded Farringdon station. Photo: Network Rail.