More than 3,690 new plastic sleepers were laid during engineering work to renew the track which was successfully completed on Tuesday, February 27, following three short closures of the line during February.
It's the first time that composite sleepers – which lay on top of the ballast, hold up the rails and keep them the correct distance apart – have been used at such volume anywhere on Scotland's Railway.
Trains couldn't run while work took place but have now returned following the £6m investment in the project, which will keep the railway reliable for years to come.
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During the work, 10,860 tonnes of ballast, around 6000m of new rails, and 448 concrete sleepers were also laid on both lines.
At over 1000m long, Moncrieffe tunnel - which first opened in May 1848 - is the fifth longest and one of the oldest railway tunnels in Scotland.
Craig Barclay, operations director, said: "The work we've completed at Moncrieffe tunnel will keep the railway, and train services, as reliable as possible now and in the future.
"While the tunnel has had a long and very significant history, we've used innovative and modern technology that'll keep trains moving on what is a vital route on Scotland's Railway.
"We thank passengers for their patience while we carried out the work."
The recycled composite sleepers will help Network Rail in its aim to achieve a net zero carbon target by 2035.
The sleepers also offer a significant life cycle of 50 years and when they are eventually replaced, they can be re-used, re-purposed or recycled to make new sleepers or other composite products.
