Network Rail completed the work which was a 12-week phased programme.
All lines through the key junction on the West Coast main line are now open and operators will again be able to run a full timetable of services, including over weekends.
It was part of a £164m Scottish Government investment to fully remodel the junction, making it more reliable and better able to cope with future passenger and freight demands.
New track sections, overhead lines, signalling and telecoms were installed, with improvements to station platforms, embankments and drainage across the junction.
The logistical challenge was immense with safety the key priority. Around 300 people and a mix of engineering trains, rail vehicles, construction plant and machinery all carried out work on site on a daily basis.
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Throughout the closure, Network Rail and freight operators kept supply chains moving on the railway rather than by road, reducing the potentially negative environmental impact.
This involved diverting around 1,000 freight trains, equating to some 360,000 tonnes of goods, to keep store shelves stocked and production lines operating.
Jim McCLeary, Network Rail's project lead for the remodelling of Carstairs Junction said: "Closing such a key junction on a critical cross-border route has been a massive undertaking for everyone involved in the project - and for the rail industry generally.
"Incrementally over the 12-week period we’ve re-opened the new, modernised railway at Carstairs - delivering greater reliability, flexibility and increasing line-speeds to this critical junction on the West Coast main line.
"The volume of work achieved during the line closure was huge and could not have been delivered practically or efficiently in any other way.
"By completing this crucial upgrade work we have improved the capabilities of the junction, providing better future journeys for passengers and more capacity for freight.
"We're grateful to passengers for their patience during this work. We appreciate it has been inconvenient but with services now returning to a full timetable, we hope that passengers enjoy the benefits this investment in the railway has delivered."
