Works under the contract should be completed within three years, NRIC said in a statement on Monday.
The project benefits from EU funding under Operational Programme Transport and Transport Infrastructure 2014-2020.
The upgrades are part of the second phase of the project for rehabilitation of the railway line connecting Bulgaria's second largest city Plovdiv with the city of Burgas, on the Black Sea coast.
(1 euro = 1.95583 levs)