A 1,200 tonne crane was needed to hoist the structure's four 107 tonne beams into place. Upon completion, the bridge, which will eventually span more than 70m.
In a statement, the Borders Railway has said that the final phase of installing the structure will take place in two weeks' time.
The work at the weekend came after seven months of preparatory work where the bridge's piers were built on site.
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Some 30 people were employed on site to lift the supporting beams into place.
Transport Minister Keith Brown attended the installation.
He is quoted by the BBC as saying: "The building of the bridge at Hardengreen is a real symbol of the resurrection of this railway line and all it means for the people of the Borders and Midlothian - new job and investment opportunities, social benefits and increased tourism.
"Around 400 jobs are being created on this project, which is employing up to 1,000 people at any one time.
"Work is now under way all along the 30-mile route of the Borders Railway and we are really gathering momentum towards the opening next year."
Phase two of the project works will see the installation of the bridge completed and will take place across the weekend of 28 February to 3 March. Hardengreen roundabout, the A7 and local roads will be closed as a result of the work.
(JP/IT)
