The route is now open to passenger and freight services since this morning, 22 February.
The Lamington Viaduct, which runs over the River Clyde, was damaged due to extreme weather caused by Storm Frank on New Year's Eve.
An intensive seven week engineering programme followed to stabilise the structure.
Floodwater left the second pier on the brink of collapse after floodwater out much of its foundations.
The incident also damaged the second pier's steel bearings, which support the bridge-deck and track above the pier, a non-load-bearing section of the viaduct's third pier and the structure's north abutment.
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Over 7,000 tons of rock have been placed around the battered structure, while concrete supports have been developed around the damaged second pier and anchored into the riverbed using 100 8cm-long steel rods.
Works have also been undertaken to replace the bearings on the second pier and repair the damage to the third pier and north abutment.
Activity is set to continue with work to re-profile the river banks and re-shape the viaduct’s piers to further improve the flow of water through the structure.
Phil Verster, MD of the ScotRail Alliance, said: "I am delighted our engineers have been able to complete this vital job earlier than scheduled and get passengers back onto the West Coast Mainline.
Claire Perry, UK Government Rail Minister, added: "When I visited the Lamington Viaduct in January I saw first-hand the scale of the engineering challenge and the dedicated Network Rail team working round-the-clock to resolve the damage.
"We can now get rail customers and rail freight moving again on this vital cross-border rail link, which is the western backbone of the network."
(LM/MH)
