Engineers worked 17,000 hours to complete upgrades on the 183-year-old Clay Cross tunnel as well as the Milford Tunnel, both on the Midland Main Line.
Over two miles of track were renewed as part of the scheme, in addition to drainage upgrades, new sleepers and specially coated rails that will make the tunnels more weather resistant.
Between both schemes, a total of 32 engineering trains were used to replace 5040 sleepers, 12,322 tonnes of ballast and 360 tonnes of rail.
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Network Rail worked with East Midlands Rail and CrossCountry to arrange bus services and rail diversions for passengers affected by the works.
Commenting on the upgrades, Steve Hopkinson, Operations Director for Network Rail in the East Midlands, said: "It is great to see that this essential work to fully upgrade the tracks in these two Derbyshire tunnels has now been completed.
"I would like to thank everyone for their patience whilst we carried out this significant programme of work, which will bring much smoother and more reliable journeys to our passengers and freight users travelling through the two tunnels."
Reflecting on the work, Clay Cross North County Councillor Charlotte Cupit said: "Many thanks to everyone who has been involved in this major local project. During the works, it was a privilege to be able to visit Clay Cross Tunnel to see this historic site and to learn more about the upgrade to a key part of our local rail infrastructure. Thank you to Network Rail for organising this."
