Services on the West Coast main line were severely affected in September 2016, when a train derailed after hitting a landslip, caused by torrential rain.
A train travelling in the opposite direction then clipped the derailed train.
Although nobody was injured in the incident, Network Rail have spent the following 14 months and £3.954m stabilising the cutting, and removing vegetation so that 1km of rock netting and soil matting could be installed on top of the 500-plus rock anchors which were embedded into the chalk face.
The West Coast main line runs from London Euston, and includes stops at Cheddington, Leighton Buzzard, Bletchley, Milton Keynes and Wolverton.
Gavin Evans, scheme project manager at Network Rail, said: “Passengers will have more reliable journeys through the Watford area thanks to this investment.
“The challenge was to work safely around the operational railway without impacting on journeys.
“A large proportion of the work took overnight and at weekends and the work we have done will protect the cutting for more than 60 years.”
Steve Helfet, London Midland’s head of West Coast Services said: “The work was done with minimal impact to our customers on one of the busiest stretches of railway in Europe.”