The bridge has been replaced as part of a wider £140m Scottish Government investment in the East Kilbride Enhancement project to decarbonise passenger services.
Thornton Road bridge is one of several along the East Kilbride to Glasgow railway corridor being raised to provide more space underneath.
This will allow engineers to install the required overhead line equipment so that electric trains can run on the route.
Network Rail, along with contractor Story, started work in December 2023 and the bridge is the second to be completed after a new footbridge near Hairmyres station opened last month.
Simon Humble, Network Rail's project manager for the bridge works, said: "The team's made incredible progress since starting the bridge replacement works towards the end of last year. This hasn't been without its challenges given the very wet and windy weather, particularly over the last few weeks.
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"The re-opening of Thornton Road represents an important milestone on the enhancement project overall, as it marks the final element of our bridge replacement works in East Kilbride.
"I would like to thank the community for their patience both during the delivery of this work and the on-going activity across the wider route.”
After the old bridge was demolished at Thornton Road over the Christmas period, the team focused on building the new one by craning six precast concrete units into place that each weighed up to 35 tonnes.
Elsewhere, progress is continuing at the site of the new Hairmyres station, where the team recently installed 184 concrete units to create two new platforms.
At East Kilbride station, work to extend the existing platform by 51m is nearing completion. This will accommodate moving the station buffer to allow the construction of the new station building.
In addition, a 14-month programme to replace Thornliebank Road bridge in East Renfrewshire started earlier this month, with the old structure due to be demolished in the autumn.
Electric services will be introduced on the East Kilbride line following completion of the project in December 2025 and will offer enhanced operational performance and flexibility of service on the route.
